Why?

[BillyKaplan]

unrealistic, but soloed regardless. why give instances the treatment and not the overworld?

1. You're totally ignoring it but the game forces you to play in groups for the vast majority of it. Tons of content can be ran unsycned and you'd still need a party for it. So no, unsync isn't the be all end all solution to group content, especially since you can't even unsync most of it when it's relevant.
2. Because you're not influencing anyone's gaming experience when you run content solo'd. You do when you're running around in the overworld. That's the difference.

fates can already be soloed, normal ones.

You'll notice those are the ones that give you dirt for awards, too. And funny you're hanging onto FATEs because FATEs either sync you down, or still require an army to clear even at 70 (hi Odin)