Two reasons.

First, even if you perfectly know your suitability for the job, it's (perhaps unfortunately) true that interviewing itself is a skill you can improve at. Feedback is in general a great way to improve faster at things, so I think it's natural to want to know why you were rejected.

Second, if you were rejected for a reason out of your control that would inform what types of jobs you apply for in the future, so also useful knowledge.

I agree in the end that you should accept that you're not going to get much feedback, but it's totally reasonable to want it.