Aside from "lawmakers don't/won't understand the tool", why not do it this way?
However, the draft stage isn't documented this way. Members negotiate whatever between themselves (well, really their staffers) and this happens over email, in discussions, via Word documents - whatever works.
I guess in the git metaphor, drafts are in flux while being worked on as a commit, and are squashed and then accessible as such squashed commits once initially introduced or whenever they lead to bill amendments. You can't necessarily track down what member was responsible for a specific sentence in an amendment.