Because it's not so cleanly cut. Editing a level might touch several files without you realising, you might move a mesh file which causes 100s of references to update across 100s of files.

For example, you want several people collaborating on a level. Say a level designer, an artist, a lighting person, audio. Usually you split this into several files to allow for the collab but you're bound to hit moments where you need to edit across the board. The level designer moves an area which causes the light, audio, meshes to move. If you don't have a way of knowing someone else is working on those files you're going to step on a lot of toes.

Whether this should live separately from source control is a fair question, but it's just a simple place for it as it's tied directly to the files.

Also, the outcome of a mistake is quite high, it could mean a whole day or two (a week?) of wasted work because you didn't realise someone was also editing the same file.