Probably because if you and me would write one and install it on our cars it would void all certifications and make the car not legal to drive. That doesn't mean that manufacturers could not band together and make a common OS for cars, or a company in that market could not sell its software to everybody (like MS or Google) but I believe that manufactures don't want to completely commoditize cars and go the way of gas brands or smartphones. A car is 4 wheels, steering and brakes to me, so I don't care much about what I'm driving as long as it handles well and brakes strong, but that's not the case for most people so manufacturers want to add their own bells and whistles.