Because balancing. It starts getting a lot harder to tweak all the numbers when you've got more skills available while synced than you did when you were originally that level.

A BLM synced to level 20 with level 20 skills only needs to be balanced around a level 20 BLM.

A BLM synced to level 20 with level 80 skills, and now you've gotta figure out how to rebalance everything they have at 80 to be equivalent to what they would've had at 20.

The high level player with all their skills needs to play much more optimally than the at-level player just to meet the same damage output.

Any rotation flubs or missteps will be significantly more detrimental to the high level player.

And before anyone says "oh, well, that should still be easy, just tweak those potencies!" Ok. That was one example, at level 80. Need to have a different balance at 79 before Xenoglossy is part of your rotation. And another balance at 77 before Enochian gets boosted. And again at 75 before Umbral Soul. 73 before Enhanced Sharpcast. Etc etc etc. Balancing and adjustments needed every step of the way, at any point your rotation changes or improves, just to keep it all equivalent to the level 20 BLM.

Or. Instead of allllll that work. They could just sync you down to actually be level 20 and make everyone's lives a whole lot easier.