1. Homework should absolutely be graded to provide students with feedback, assess learning progress, and tailor instruction to strengths/weaknesses of students in the current iteration of the course. Once all the work goes into grading, it's hard for teachers to throw out that data when assigning grades.

2. It mimics the real world where sitting your butt in a chair and turning in useless reports is valued -- perhaps not as much as actual outcomes, but still valued. Perhaps that's stupid, but in most jobs it's a reality. IMO this isn't a great motivator, but it's one a lot of teachers cite.

3. Helps pad the grades of students who are terrible test takers / have anxiety issues.

4. Students are... students. They don't always know what's best for themselves. Making homework worth a portion of the grade communicates to students that they need to practice. Patronizing? Maybe. But also a reality.

5. It's the easiest way to inflate grades without explicitly lowering standards.

Incentive to do it.

It shouldn't be, and isn't in many countries.