It’s inherent to the way writing prose works. Documents are sequential, not modular. You can’t have 10 people all working on a novel at the same time. Maybe there are a few logical sections you can split up, but even then everything needs to be in the same voice so someone needs to massage the pieces during integration. Automatic merging provides little benefit over copying and pasting a couple of individual sections together. There are also very few documents being written. (The ratio of documents read to documents written is probably 100:1 between cases, discovery documents, etc.) You’re never editing dozens of files at the same time. Maybe 3-4, and often just one.