Why can't you complete quests higher than your level?

[Moontide]

Nope, most optimal leveling for subclass is by doing FATEs and doing the SUB QUESTS as you don't have the main quest for it anymore, while in your first class, it's Main Quest + FATEs.

That's sort of beside the point though, isn't it? I was talking about what was likely to be done, not what's optimal. I doubt all that many take the cookie cutter content-skipping route anyway. People usually need the gear from quests first time around.