Momo Sama has a
very well written explanation for why they can't do this.
In short, because of how SE has set up the Prange role, buffing Machinist up to Black Mage/Samurai levels instantly kills off any reason to ever touch Dancer or Bard. In an average party setting, Machinist actually does decent, albeit still on the low side. Meanwhile, Bard and Dancer are entirely dependent on their party. In other words, a Machinist doing say, current Black Mage damage, is always 100% better in PF than Bard or Dancer purely due to consistency. Not only do either one of them have to be above 90%. The party itself/Dancer Partner also needs to be good. We're starting to see this unfold with the Caster role right now. Black Mage is, objectively, better than both Red Mage and Summoner. What prevents it from dominating is the general difficulty of playing it well and combat raise.
SE has essentially pigeonholed themselves into a corner ever since they introduced Dancer. At this point, they need to simply scrap the whole "selfish" concept from Prange.