That would make some things more difficult. For the same reason we use Git to be able to work on multiple things at the same time.

Let’s say you have tax law and the particular law is thousands of paragraphs long. You have multiple changes to the law going on at the same time (totally unrelated stuff in different parts of the law). It’s going to be really hard now because each of the 15 changes need to update its own texts as soon as any other of the proposals get changed and it’s going to affect exactly which order they are voted on etc.

Much easier to just say propasal A changes paragaraph 33 and proposal B changes paragraph 475