Jim Davis is the nom de plume of a collection of artists who work together to produce the comic strip Garfield, a strip about a cat. Even though there IS a physical Jim Davis that started it, at this point, it's a pseudonym for a gaggle of about 40 people.
That's literally it.
Some syndicate people sat around a boardroom and were like, "cats are popular, just do a strip about a cat". The goal being not necessarily to make people laugh, or to advance the art of comics in any way - just to push as much merchandise as possible, to cat people. And we are cat people, judging by the sheer number of cat photos on the internet, and LOLcat memes. The suits were right, and they raked in the cash.
Garfield is a fat, sarcastic looking ginger tomcat owned (or is it the other way around) by a complete loser named Jon Arbuckle - and their interactions are designed solely to provide catchphrases or memes to place on T-shirts, plush dolls, and comc anthololgies. That's basically it. As has been noted elsewhere in this thread, the backgrounds are spartan in the extreme - usually Garfield, Garfield and Jon, or Garfield talking to a mouse or spider. As the epitome of the "anything that isn't worth doing at all isn't worth doing properly" mantra. well, they work back from a punchline to the barest of setups and draw just enough to make the joke work.
Garfield isn't just a cat, he's a psychological reflection of the kind of single cat lady they were targeting with their strip, a demographic with just enough bleedover to the rest of us that we'd find the merch as appealing. He's like a cat that he eats too much, sleeps all the time, and like many domesticated cats really could care less if a mouse runs amok in the house. As for spiders, he'll kill them on sight, because it's the thrill of killing something without any work. But he's like the demographic they're going for in that he likes pasta and carbs (his favorite food is lasagna, even though cats in general can't live on plants), drinks coffee by the hoddle, and hates Mondays. It makes no sense whatsoever that a cat would like or hate Monday because like every other cat on the planet, he's unemployed and happily sleeping in patches of sunlight throughout the day. Likewise, I've never known a cat to remotely touch any liquid but pure water, suspicious of anything supposedly drinkable with an odor.
So having established that this cat wouldn't move if you put 10,000 volts through him, sleeps all day, and hates everyone and everything, it's pretty difficult to actually chain a strip around him - given that "drawing sleeping cat" has comic potential. So they put the burden of that on Jon. Jon is, to be quite blunt about it, a lunatic. In an effort to not be a complete dating failure and an uninteresting, sad and soitary creature he decides to do any of a number of whacky things in the background that Garfield can break the fourth wall and comment on. This hasn't been thought out well - in fact, there's a wonderfully surreal online riff on Garfield which Jim Davis absolutely loves called "Garfield minus Garfield" in which Jon simply talks to himself in the house, and does incredibly strange things while alone in the house for no sane reason, as Garfield has been removed from the equation. I mean, without Garfield there to interact, Jon just comes across as a nut case. And since a cat really can't interact with you, that's in essence what Jon is, regardless of any bon mots that Garfield can add to the situation.
The only possible other things Garfield could interact with are a dog with the intellectual capacity of wet cabbage, and a stuffed bear. So the strip really doesn't do much except repeat the same things over and over. Getting fat is good, Mondays suck, I wish I could sleep forever, and men are annoying.
That hasn't stopped the strip from operating continuously for decades, keeping on its treadmill of crushing every possible red cent out of the premise.
Internet cartoon meme sensation Pusheen has pretty much done the same thing - only realizing it's dealing with a cat, it makes no attempt at dialog or any other interaction further than the cat eating pizza, or kneading someone's back, in a tiny, shareable animated GIF.