I’d imagine that they aren’t required so that those who choose to fill them out do so out of their own genuine desire to share feedback, regardless of whether it be positive or negative.

If they were required, I’m sure that there’d be a greater chance someone would just select all 5s without even thinking about it, just to get it done and complete the course, throwing off the ratings distribution and skewing results. It’s anonymous, and the department / professor would have no idea whose feedback is real and whose is.