FCP renders the same content over and over. Why?

I have a new mac studio and was hoping for better performance compared to my old machine, but FCP is constantly rendering. Yes, I know you can turn background render off, but that's not the issue. Why can't it render an edited sequence once, and just use that rendered file from then on unless I re-edit the sequence? I'll do a project, and it will fully render. If I come back the next day, it renders it all over again. Over time my rendered files folder in that project will get huge, 600GB or more, and I just have to manually delete it because my hard drive is full. Then I get to render it yet again. What gives?