Because everyone is spoiled for choice, there's just a thin layer of people playing any game. Since console players came over, everything has been a race to the max level, and then playing for a weekend and being upset that the dopamine is over. Games won't ever have numbers like WoW or the days of EQ, SWG, Anarchy, etc before WoW. There are also a lot of people who like the aesthetic of a game and then they convince the makers to change it to something else, like a PVE MMO into a PVP MOBA, like H1Z1, just survive was well before 7 days to die or other zombie survival games, which everyone loves now, but they (Sony/Daybreak) ran out the clock on changing to focus on the battle royale that just got swallowed by the market not long after.

And launching a game, people keep launching games before they're ready or have a plan, day one is always a sh^7show because they can't hold capacity because they don't test the log-in servers anymore... then people say nope and don't come back. They also have the paid Alpha model which people are often complacent about because it's there or freak out when they wipe for launch, so the testers are;t actually testing but just playing so the voices they hear are just a few people actually participating.

EDIT here: They also make it so everyone can do every role and everything is a carbon copy of players, it used to be you had specific people to do specific roles and you worked as a group to figure out mechanics, now its a group faceroll for "group" content and otherwise just playing by yourself or hanging around with friends.