They travel for different reasons and if they keep on doing it is because it makes sense. Certainly, there's plenty of money in NYC!
You recognized several escorts in the sling giving it for free? Even if you have such a memory that you saw "several" in the sling "giving it for free", they possibly were hired by the club to be there.
Maybe NYC is special but usually guys in sex clubs are not the most attractive ones. Possibly guys are moving away from Grindr after wasting so much time trying to get laid.
Slowly moving away? For a $20 you can go to a club and get any guy you want for free just do it.