[thanatropism]

One reason is diversification.

If there are N ways to write a program, M of which are security hazards, it's better to have M/N of all programs exposed to risk than have a M/N chance that all programs are borked.

"Reinventing the wheel" is a leaky cliché: the problem at hand isn't that people would independently try to come up with the solution to a simple problem (rolling something down a hill), but that the instantiations of a solution would be irrevocably linked, such that one flat tire stops all cars.

What's more - jesus, go outside, look at how many kinds of skateboard wheels, car wheels, bike wheels you see in five minutes time.