The reason is because they view each expansion as a "whole new game" (their words). When you sync down, you sync down to the "rotation of that expansion". Now, sure, they remove jank and merge things to reduce the overall amount of buttons at those levels to make it manageable at higher levels, but the essence of the rotation is mostly intact. If they revamp it, they still try to maintain some similar essence to what it had at those levels ie. you still use egis at lower levels on Summoner, you still spam the Rage of Halone combo on PLD at level 50 and it still has no magic attacks until Stormblood.

So if they synced it by adjusting potency they would need to have a button that level syncs the attacks still in the settings, so that we can play the rotation of those expansions if we want to.

There are balance issues with it though. Content is designed around jobs having certain abilities, which is part of the reason for homogenization. For example, at a certain point you notice all tanks get a "short cooldown" that is less than 30 seconds and then that becomes an integral part of mitigating some fights. At a certain point we get more heals but then they scale outgoing damage accordingly. And lower level content may not have necessarily been designed with these evolutions in mind. It's also difficult to predict how each person's personal level sync could unbalance lower level content in terms of damage with an automated, one-sized fits all potency sync, when we already unbalance it enough as it is.

As for them not putting money into the game, I really think it's just decision-making. Their focus is always on the next patch, and nothing else (except the Graphics Update, which there was notably few updates to in the last patch if any which makes me wonder if there was a problem).