The issue upstart MMOs face is that players have come to expect a LOT of content - or else the game feels empty and “dead” by their standards. The existing MMOs that have powered through for all these years didn’t have that issue since they came out a long time ago - 16 years ago, in WoW’s case.
Nowadays… imagine a new developer trying to create the mountain of content players expect from an MMO, only to risk having it flop because it wasn’t different enough from its competitors. They can add unique stuff - but as Blizzard has shown us every recent expansion, unique isn’t always good.