I've been looking at cheap, low-end movie cameras recently like the Blackmagic Pockets, Sigma FP etc. I'm sure this is a naive question but I've been wondering

I've been looking at cheap, low-end movie cameras recently like the Blackmagic Pockets, Sigma FP etc. I'm sure this is a naive question but I've been wondering why I can't buy a good sensor module, attach it to a Raspberry Pi or similar, put it in a nice case with an SSD and a lens mount etc and call it a movie camera. I guess that's kind of what Apertus are trying to do with the Axiom, but they still don't seem to have shipped anything real after quite a lot of years. The low end of professional-ish cameras is great in lots of ways, but they all have annoying trade-offs. To be fair I'm sure any janky POS I built would too.