Because if it's in-process, all that means is that you have to ensure you have a single writer thread. There's nothing wrong with that design; it's how a MongoDB node works internally IIRC.

Granular locking is for the convenience of multiple pieces of code that don't know about each others' existence. Within a single process, it's not really that big a problem to ensure that all the relevant pieces of code do know about each other, and collaborate.