It's easier - for a skilled GM - to produce engaging new content for a tabletop RPG than it is to produce the same for an MMORPG. The GM only needs to come up

It's easier - for a skilled GM - to produce engaging new content for a tabletop RPG than it is to produce the same for an MMORPG. The GM only needs to come up with an idea that works for their group of - say - 4 players, whereas the MMORPG vendor also needs to come up with artwork and code and ensure that the content is balanced for thousands - or more - of players at different stages of development and with different playing styles. MMORPGs feature grind because without it the players would run out of content before new content was available: then they'd get bored and quit. Or, if the content was produced quickly enough, it would likely lack sufficient quality control and players wouldn't like playing new content, with the same result. Of course, the more money the vendor is able to put into creating new content, the less grind the game needs to depend upon. And if the game has many social players, their social interactions provide another mechanism to keep players interested after they run out of content. Eve is a good example of an MMO where player interactions are a big part of what keeps players in the game.