Lawyer here: because your engineer's POV is wrong. I don't know what else to say. It comes from a lack of knowledge of how lawyers actually work and therefore what we actually need for our work. It is not identical to what software engineers need or want for theirs.
The drafting process is the area in transactional practice that least needs "disruption". The process is long-settled and fairly universal. Track changes are not "crazy confusing to parse", they're a simple and easy way to know who edited what and when. Version control? Basically meaningless for lawyers. The only version of the contract that matters is the most recent one, so we just need a way to track that. In actual practice we don't just revert back to some older version; that doesn't even make sense to someone who understands what we're doing.
What really needs disruption, or at least improvement, are contract management platforms. I've worked with all the major ones and they're all pretty bad and clunky in their own way (IME Ironclad is the least-worst but still leaves much to be desired).
If someone could develop a simple, intuitive system for extracting and recording certain agreement content and making that searchable (and transferable!), as well as storing and searching agreement documents generally, it would be amazing.