Why do I always have to reinstall everything.
Because Windows?
Windows has no good way to verify everything is correct, not sure any OS really does.
And you don't
have to, but if something is wrong on install you will NEVER be free of it until you do a fresh install. It won't fix itself unless you can do a CRC check of every file and I'm pretty sure the OS itself has no way to do that. There's an old computer saying,
garbage in garbage out, but it should also be said start with garbage end with garbage.
Doing a fresh install has also loooooong been known to be a cure all for many of Windows ills. It's why whenever you send in a system to be repaired they do a wipe, they need to start with a known good configuration. We've moved away from that some but the reasoning still stands.
I tried doing memtest for a while and I do not get ram errors anymore, even with four sticks. Is it possible a misseating of the ram could have caused the bad memtest results earlier?
It's possible, it's also bad cpu mount, modern cpus are known for dropping memory channels if not seated perfectly flat.
And if not, is it possible I can just go by new ram sticks and hope things work?
There's no guarantee the new stuff will be any better.
And in case it is bitlocker... Is there no fix to it?
You still haven't determined that was at fault and if it was, it could have been a result of memory, not Bit Locker. A re-install of Bit Locker itself after the memory fix would probably fix it. IF that was the only issue.
Is it a coincidence the crashes almost always happens ONLY when I am not using it? It can happens like 10 minutes of not using the PC, but it does not happen while I am using it per se.
No, but it is a telling thing.
Certain things run while idle such as drive scans for Windows search, TRIM, defrag, cloud storage uploads, etc...
I have reinstalled my pc 15 times the last year or so already
You should be good at it then.
Welcome to bleeding edge Windows life, keep at it and you'll soon be doing backup/reinstalls in under an hour (minus game reinstalls).