When cars were relatively new, automobile racing rules didn't really exist. People could just "run what ya brung". But cars weren't even twenty years old before people realized unlimited racing really didn't quite work. Racing is about competition, and it was always possible for someone with a fat wallet to come along and build a bigger hammer. The competition becomes one of wealth, rather then skill, and unlimited expense very quickly drives out most racers. Race promotors have a different agenda. They make their money selling tickets, and once-sided races don't sell very many. Racers generally agree with promoters on this point, but for different reasons. Racers want to compete on the track and in the shop, not at the bank. They want a level playing field where they can showcase their ingenuity and skill. If they're amateurs, they just want a place to race where they won't be humiliated.
Thus was born the rule book. In many cases the rule book was written by race promoters. That works pretty well for local, amateur racing. In my home, Columbus, Ohio, there is one asphalt oval, Columbus Motor Speedway. CMS has NASCAR sanctioning but the local rules are essentially chosen by the track, in consultation with local racers. As traveling is expensive and sucks a lot of time, most local boys race there or at the local drag strip. Even in road racing where tracks are regional, many local racers never travel farther then Mid-Ohio Sports Car Course. Professional racers want to race at all the cool tracks, so they need a sanctioning body. Most stock car racers have adopted NASCAR as their sanctioning body. The National Hot Rod Association was created to regulate drag racing, the Sports Car Club of America (SCCA) regulated first amateur, and then professional road racing. The United States Auto Club (USAC) sanctions open wheel races on oval tracks, most notably the Indianapolis 500. The FIA sanctions Formula 1 and most international racing, but IMSA sanctions sports cars in the U.S. There are other sanctioning bodies, some of whom sprang up as protests against this or that rule, as racers tend to be strong-willed sorts.
Some of these bodies co-operate (for example the SCCA and FIA recognize each others licenses) but the most important relationships come between race promoters/track owners, racers and the sanctioning bodies. They have to find a way to get along. Championship Auto Racing Teams began as a white paper written by the great Dan Gurney. USAC gave its track owners a lot of freedom, but the racers (led by Gurney) wanted to run a race series on both ovals and road courses. Each USAC track had its own rules, especially the Indianapolis Motor Speedway. Gurney and other racers did not want to have a build a different car for each course. So they went on and formed CART as a racer-run sanctioning body, until IMS owner Tony George blew it all up and took the Indianapolis 500 away from them. There is always a natural tension between track owners, racers and sanctioning bodies.
But at the end of the day all of the above need to get along if racing is going to work. Sanctioning bodies and promoters need to pay attention to their racers problems. Racers know that if the fans don't turn out, neither will the sponsors who actually pay for their racing. For professional series like F1 or NASCARs cup series, the sanctioning bodies command a lot of money, the big races and a lot of power so a lot of what they say goes. Bernie Ecclestone can be a little Napoleon, but teams won't pull out because F1 is the only game in town. NASCAR enjoys similar power with a less imperious style. But if you don't have that level of power and money then you damned well better listen to your racers or they won't show up. And if your a club, like the SCCA you are your racers. I've known several members of the comp board, and they all raced. So there is necessary and formal process for racers to petition for change. The result is rules creep.
An excellent example is the Production category of race cars in the SCCA. The SCCA started out when people started racing the MG, Triumph, Porsche and Jaguar sports cars for fun. In the early days you had to own such a car to join. Racers want to win, it's part of their DNA. Tuning and setting up a stock car helps, but sometimes it helps if you give the motor a little extra. Creative rules interpretations (sometimes known as cheating) became common and the guys who really had stock cars started protesting. The SCCA comp board met and the rules were changed.
Keep in mind the SCCA was formed in 1944. For over sixty years now the various Competition boards have had to hear formal petitions from members in good standing (who may be personal friends) arguing that if only the club would make this change racers could go faster (yeah!), race safer (double yeah!) and save money (racing nirvana!). While often sincere, experience showed that said members in good standing had often figured out that if you let them do what they said they wanted, they could then use the new language to do something else, which cost even more. Production cars began as well, production cars. By 1999 a nationally competitive production car has fiberglass or carbon fibre body panels, altered suspension mounting points, an engine which revs to twice its original design specs and produces three times the power, transmissions where the only stock features are the case and number of gears and so on. These 'production' cars became 'very highly stressed' race cars which required constant maintenance and hideous expense. So much so that by 2003 the category was failing, and a new less-expensive 'limited prep' series of cars was introduced to revitalize the category.
Nor is NASCAR immune to rules creep. Once stock cars were actually stock. Modifications crept in pretty quckly though, and like the SCCA NASCAR chose to accept reality. In the 1960s a stock car used the stock tub of a real production car but with extensive modifications and the active participation of both Ford and Chrysler's engineering department. Later on they switched to a tube frame car because although tubs are cheaper to build initially, tube frames are cheaper to campaign in a professional series. Nowdays a 'stock car' is a fully tube framed car whose body must fit a NASCAR defined shape template which often had only the slightest resemblance to the actual stock model it was supposed to represent. Front wheel drive models race in rear wheel drive form. Overhead cam sixes gave way to overhead valve V-8s. Fuel injection is new to NASCAR in 2012. The engine blocks are made from stainless steel. They may be called stock cars, but they're not.
Even at my local track, the entry level classes in stock car racing are very, very highly modified cars whose unit body is still present, but has given way to steel tubing for real structure. The fastest classes clearly have almost nothing to do with a stock automobile.
In racing, rules change over time in response to concerns over safety, costs and competition. Racers will never, ever stop petitioning their sanctioning body for changes. Some of these changes will be adopted, and as a rule they will push the competition toward more expense and higher speeds. If sanctioning bodies fail to take into account racer requests, racers will go somewhere else to play, unless the sanctioning body holds a trump card like the Indy 500. Because of this, racing rules will never remain static.