system
A chat on subreddit /r/NoStupidQuestions
human
The whole point of evolution is variety and its not planned.

If things didn't vary, things couldn't evolve.

It just so happens that the different blood types didn't cause any evolutionary advantage or disadvantage. At least not enough to weed the others out.
gpt
They cause a significant disadvantage.  RH negative women carrying their second RH positive child to term can die from it.