I've come to the assumption that they're using the celebrity guest stars as vehicles to keep the show relevant and popular with audiences: keep in mind, up until the 1990s, SST had a monopoly on children's educational entertainment, but since then, they've constantly been in competition with other shows and other networks out there, so they have to look for ways to fend off the competition... but I agree, it seems like the celebrity guests have become a wee bit excessive as of late, like almost every other street story is driven by the celebrity guest (Jimmy Fallon as the nature survivalist was probably the worst example of this).
But as far as the regular veteran humans on the show, Caroll Spinney has explained many times before that most of the time, because Sesame Workshop is a non-profit organization, and SST is a low-budget show that they can't afford to bring in ALL the humans very often, which is why we may only get to see more than two or three of the actors in the same episode for special occasions (Mr. Hooper's death being an example of that).
But as far as the regular veteran humans on the show, Caroll Spinney has explained many times before that most of the time, because Sesame Workshop is a non-profit organization, and SST is a low-budget show that they can't afford to bring in ALL the humans very often, which is why we may only get to see more than two or three of the actors in the same episode for special occasions (Mr. Hooper's death being an example of that).