Why does it not work for dual wielding?

because it specifically says you should have off-hand empty.

But does that not make it a completely useless feat? I mean you either use 1 2h weapon, 1h weapon + shield or dual wield, nobody uses just a weapon and an empty hand right? :-/

says in the pathfinder wiki that it does not work with flurry of blows tho so I guess only magus/duelist then? :)

tbh I haven't tested it with monk.
with how this game works it might as well be working, magus technically has his off-hand occupied with dual-casting. but it works for him just fine. pretty sure it will be the same with monk and flurry

tested it. seems to work just fine on monk.