They can't just sync stats as every new ability you learn is typically designed to increase your damage or power in some way, especially when it comes to passive buffs or oGCDs. They would have to adjust every ability's potency by varying amounts based on what level the dungeon is. An ability at 90 that has 300 potency would end up having a potency of like 60 in Sastasha to make up for all the other abilties you have that you normally wouldn't have at that level. Where a healer could get once get away with throwing the occasional Cure or Medica would now have to make full use of their kit to keep people alive due to how small their individual heals are.

This is why blue mage just streamrolls old content. They don't lose access to later level abilities and keep their potencies.