Because a skillful teacher can encourage students to want to do their homework, with rewards for the effort and not just handing in correct answers that were done by mommy and daddy or a tutor.
It's not hard at all to write assignments that students can do without their support group doing it for them. Homework should be reinforcement for what was learned in the classroom. In many cases, the problem is that the teacher fails to teach something well or at all, and the only students who "get it" are those who have someone else outside the classroom teaching it for them. And then they blame the kid for not having parents who can teach the lesson or pay someone to do so.
After I retired, I spent about four years as math coordinator at a high-end learning center. Some teachers were like gold mines for us....most of our students came from their classes and they needed someone to teach the material.
That may work better when home environments are more stable but when the chaotic home environment is characterized by blended families, loud music/TV and people chattering/yelling/arguing, how is the child expected to get anything done at home?