Homework is a tool for students. It is where they practice. I see my job as a prof to curate the problems I think will help students best prepare for the exams, and use that space to help them gain the intuition about the problems I don't have sufficient time to cover in class, like through an application. As my profs before me used to say, true learning comes in doing problems.

I find bright people who are self-motivated tend to do problems on their own, but this is not the modal student. This is like the top 10-15% of any class. The modal student is motivated by getting a degree so they can enter the working world, and sees coursework as an obstacle to that. They'll take any and every shortcut to get the highest grades with the least amount of effort. However, particularly with first and second year undergrads (but I've also seen the same behavior in grad students), they do not understand the purpose of pedagogy. That doing problems is where you learn the material.

So if you make homework optional, the 10-15% hold steady and do it but the bulk of the class will stop doing problems and tank their grades on the exams. They will not internalize the lesson after that first bad exam, because it is morally and practially cheaper for them to believe they had a bad day or that their professor (me) is just a terrible meanie who constructed an exam with problems they've "never seen before" and "had no connection to the lecture" (actual evaluation comments I've had when I've tried to the homework-optional method).