Why can't I watch all movies on MUBI?

I'm curious, why is it that I can't seem to access all the movies available on MUBI? Is there a specific reason behind this limitation? I've been a subscriber for some time now and have noticed that the selection changes frequently. Does this have to do with licensing agreements, or is there another factor at play here? I'm eager to understand the reasoning behind this, as I find the platform's curated selection quite appealing.