The main mechanic of MMOs is grind. You do the exact same thing over and over again just for a chance to get an epic item you can brag to your friends about.
And yeah, okay, let's assume the core gameplay of RPGs is exactly that. Doing the same thing (fighting) over and over again to get better items and level up. Oldschool RPGs like Wizardry, Might and Magic, Gold Box games, they are essentially focused on fighting, getting XP, and upgrading your gear. Even more involving modern RPGs like Gothic, Morrowind, Baldur's Gate - they're essentially the same, too.
Even if we assume that, MMOs are exceptionally shitty at providing an engaging experience in this core gameplay aspect. In MMOs, you spend a lot of time fighting the exact same fights and the gear drops are always randomized.
In ToEE, which isn't known for its excellent encounter design, there are nevertheless a couple of cool and memorable encounters. The fight with Lareth in the Moathouse is a great one, for example. It's challenging and memorable, and you fight it once because it's a boss fight.
In an MMO, the Moathouse would be an "instance" and you'd run it with your group at least once a week until you're out of its level range, which can take a lot of time. Most MMO dungeons are played through 5 to 10 times at least. You do the same content over and over again until it grows stale. And that's just the special dungeons you run with a group. Usually the gameplay consists of running through the same landscape you've run through a dozen times already, grinding the same respawning mobs you've been griding for the last two weeks.
And the items? Well, in a game like Morrowind you can beeline to the location of a unique item if you know where it is, and exploring a hard dungeon will always give you a certain hand-placed unique item as a reward at the end.
In an MMO, you'll grind the same boss a dozen times for a 10% drop chance of a unique item you need. If it doesn't drop this time you gotta run through the same dungeon with the exact same encounters again.
MMOs are designed to waste your time and require a disproportionally high investment of effort on the player's part. You have to grind a lot if you want to keep leveling up and improving your gear. The actual gameplay is not very engaging. In fact, it tends to be less engaging than single player dungeon crawlers from the fucking 80s.
The fun in RPGs is encountering new situations which you then overcome with the skills of your character or party. A dungeon with puzzles, Dungeon Master style. Hand-placed combat encounters, KotC style. Situations you can solve in different ways, Fallout and Arcanum style. Do stuff, get XP, level up character, find new stuff to tackle with your better character.
MMOs, for a large amount of their gameplay time, lack that "find new stuff" aspect that makes RPGs fun, because they require you to do the exact same stuff over and over again. For some people this causes attachment to their character and the game because they spend so much time with it. For me, it caused me to give up World of Warcraft after 3 weeks when I played it back in 2006. I had explored everything, done every quest in the current region of the game, wanted to progress to the next region - only to find out I was underleveld for that area and constantly died to standard enemy mobs in the wilderness. When I realized I had to go back to the area I had spent the last 2 weeks in and grind grind grind, I quit the game because fuck this shit.
