I've linked the part below [1] where Chad (ex Tesla repair technician) explains why Tesla do what they do, i.e. only replace the whole battery pack (which costs 16k) when some sub unit is broken.
The main reason is cost effectiveness (for Tesla): Instead of training staff how to disassemble, have parts in storage, maintain facilities, assemble again etc, it's cheaper to do 'assembly replacement' and replace the battery pack as a whole.