Why don’t they just make 25 the age of majority since that’s when the brain is fully developed?

I expect this to occur, especially for women. As a psychologist points out in the comments, this supposition about the “brain being fully developed” is bunk. But I expect the “age of adulthood” to be made 25-30 for women in my lifetime de facto or de jure, and probably 21 for men. But perhaps just for women — hard to tell now.