I always shout for parties for depth and have no issue finding members. Let's be real, the problem is people don't want to make parties for endgame content, they just want to join them. There are certain times when it takes a while to find enough players, but at those times there are usually just less people online in general.
Also, even if you dont have the best gear or meta class (like me), if you make the party, you can easily do the content / make the gear while not even being that helpful xD (I know coz I always died and didn't add much dps) but people didn't mind the parties and always asked when I was making them again!
Final thing, most people that can solo, also join parties if theyre offered (at least in depth 2). I do think almost everything is soloable, but i think people enjoy party play, but again, most just dont want to shout for it.