The 50 mm (abbreviated in conversation as "fifty") is the normal lens on 35mm format.
By a rare miracle, the 50 mm can be both cheap, bright, sharp and light (in weight).
It can also focus very fast (in autofocus systems) due to the fact that there is little mass to move around.
So, you are asking, why didn't my Canon BlueWonderBongoRave, why didn't my Nikon RebelliousTeenager'xploit1A come with a 50 mm ? Why did it come with a stupid 35-120 mm zoom that is not bright enough for existing light photography and will force me to use the little built in flash as a mainlight ?
Well, it turns out that the photo equipment makers have realized that the Average Low-End Photographer just wants a zoom lens. So that's what you get in low end bodies - high end bodies don't come with a lens at all.
Luckily though, the 50 mm is still a serious photographer's favourite, which is why camera makers crank it out in many models. Nikon and Canon offer three different models each of 50 mm lens.
What is best, they are cheap: rarely will a 50 f 1.8 set you back more than one hundred dollars.
It may well happen that your best light is also the cheapest.