A big part of the reason MMO combat is how it is, and how WoW popularized it, is because it's manageable with how the US internet infrastructure's state is currently (which isn't that great - and is probably about to slow down its advancement quite a bit, due to politics...) - depressing as though it may be, it's kind of lowest-common-denominator when compared to the network infrastructure of some other countries with popular online gaming scenes. That said, you might find it hard to believe, but there are plenty of people who actually like that cooldown-based rhythmic hotbar gameplay. Some games actually do really clever and fun things with it. You could make a comparison to why people play 2D platformers still, when 3D is a thing; they actually like that, rather than the alternatives.
Probably one of the better attempts that I've at making a more action and player skill based combat system in an MMO from a western developer was WildStar. Due to questionable content focus early on (trying to target the "hardcore vanilla WoW raider" group - which is by far a minority), the game tanked - and is itself now free-to-play and pretty much in maintenance mode currently - but it's still got combat that is actually quite fun. Though you could still argue that it's cooldown and hotbar based (because, well, it is), it also places quite a large emphasis on movement and evading enemy attacks manually, and also aiming your own attacks manually. I think there's a grand total of two abilities in the entire game that require you use targeting to cast them, with everything else being manual (no auto-attacks or anything like that, even). The fun of combat isn't really all that evident until you get into dungeons though, which are kind of a crapshoot unless you have a party of friends you're going in with, since last I checked nobody actually uses the matchmaking system for them.
That aside, I do think something a lot of people easily underestimate is how much of a technical undertaking it is to make a high-speed action game work in an online environment, much less a massively-multiplayer one. Lots of developers still struggle with rudimentary hitscan shooting in online games, let alone projectiles - much less more complicated and intricate hitboxes like we see in offline games. Though I haven't looked into it personally, I'd say it's likely that many of the games that purport themselves as "action MMOs" aren't really such so much as they are a convincing set of smoke and mirrors layered on top of heavily-instanced game worlds that aren't used in a way that could in all honesty actually be called "massively-multiplayer." Some games are pretty up-front and honest about this - such as Destiny and The Division - while others are very much not. That's not to say that they can't be fine games in their own right, though.
It probably isn't helping that most of these attempts at action-MMOs are coming from companies out of China and Korea, whose companies have kind of a nasty reputation for being extreme grindfests infested pay-to-win, gacha and other less-than-savory microtransactions (which sucks, because some of them actually look really cool, but then have those things in them, which kills all interest I may have had in them).
