The reason most adults can't draw/paint is related to why they can't program. The mistakes are too painful for an adult.
The reason most adults can't draw/paint is as children they were never given any feedback which allowed them to refine and improve their technique. After a while the kid gets to a point where they are completely frustrated with drawing pictures which they can see look like cat turds and not cats and give up saying "I can't do it". By comparison the majority of children grow up with a fair degree of language competency because parents will correct the child allowing them to see, and thus learn from, their mistakes.
I had a tennis coach when I was in school who said that the saying that "practise makes perfect" was nonsense, he said instead "perfect practise makes perfect". He never hesitated to say if your technique sucked and I was a visibly better tennis player for his coaching.
Do not seek to follow in the footsteps of the wise. Seek what they sought. -Basho