if the teacher takes time
One thing they don't have.
acquire a skill by simple looking (a lot of the time - I certainly didn't)
Same here, however that's the reality of a lot of kids' lives. Kids have different levels of intelligence, different sorts of family and so many distractions. Homework certainly has value however not as a key component of a curriculum. We have to deal with the way things actually are rather than the way we think they would be in an ideal parallel world.
unless parents are stupid enough to do their child's homework instead of the child
That is far more common than you'd think. Of course plenty of parents equally couldn't care less if their kids did homework or not.