I am sure Blizzard commits an absolutely massive amount of time to balancing actually. The problem is that WoW has become so focused on perfect balance, and the classes need widly different things to be good, enough so that the best they can do in most cases is tune that aura they added in Legion that simply changes how much damage is done.
You have to consider for instance not just the numerical values, but also things like how some classes are more bursty, some classes being more dependent on stats, certain fights being better for classes and how some raids might have more of that kind of fight.
Combine all these factors together and you end up with some classes being poorly tuned. Now Blizzard could always just tune the flat % knob more often, but I think the playerbase would react far more negatively to that than anything.