Also, there's the fact that there are so many grand stories to tell in a fictional universe. If "epic" events are happening in it all the time, then they're actually humdrum, and the stakes are pretty low, so the audience has no reason to care. At the extreme, you enter soap opera or monthly comic book territory, where the writers have to come up with new storylines for decades on end, and the result is that nonsense and self-parody becomes normal, because you've exhausted every possibility in the space of reasonable stories. Characters become empty costumes with no personality (because every possible story with their original personalities has been done), the same sequence of beats gets recycled over and over with new characters and props slotted in (the Star Wars sequel trilogy is a notorious example of this), etc.

And all this happens because, at the margin, new content is still profitable to make, but every piece of new "content" (gag) drains a little bit of the magic out.