So why not lean into it fully and make the GM responsible for the whole balancing?
Because having things balanced properly in regard to the myriad options that are possible in people imaginations is hard, especially related to combat. Improper balacing leads to people having a bad time, while having an established, fair ruleset lets the DM and the players focus on other things.
No need for the framework to do balancing, because a good GM will do that.
But at this point why even have rules? A “good GM” can just entirely improvise a system.