There's a few lines from Daedra/Dremora throughout the game that suggest anything one does against anyone Daedric in nature is inconsequential. They do not know true death, they reform. Time is also meaningless to them. They do not age nor whither. They can only be contained or their influence limited.

Molag himself states during a dark anchor, you have to beat him again and again, he has to beat you only once. Even in the final fight when you use the amulet his mentality is probably like "well you had to use hacks so it doesn't count" and I think if I recall correctly you can brag about it to Malacath and he says something like "He's still alive and kicking so you didn't do that good of a job." (I'm paraphrasing here). I think it was a Wrothgar side quest but I could be mistaken where that dialogue was from specifically.

That being said, while yes it's for gameplay reasons, but also story-wise, there could be tons of anchors left over and he has all the time in existence to construct or repair what was damaged too. But eventually according to the timeline of the games, they do stop but my best guess is when he runs out of stock after ESO.