Well, I could have written this 25 years ago... For the first year, it was funny. For instance, Pheobe’s line. “Oops, did I share too much?” is actually hysterical the first time it pointed out that there is such a thing as sharing too much. But then that was Pheobe’s entire character: someone who emerged from a shocking past clueless about how shocking it was. Ten years of sharing too much does not generate ten years of laughter.
The first season of Cheers consisted chiefly of character expositions of each main character, circling around to most of them a second time, to see how they interact with each other. Seriously, go back and watch and you can see, “Okay, this is the Diane episode.” The cast was large enough that there were endless permutations. (Diane, Diane and Sam, Diane and Coach, Diane and Carla, ...) And when at last it ran out, new characters came in to give new insight or evolve the characters (Frasier, Frasier and Diane, Frasier and Sam, Frasier and Sam and Diane...). But there was always a fresh character exposition.
Not so with Friends. It was always the same characters treating the same other characters in the same way. New characters didn’t add to complexity of existing characters; they just added shock for the existing characters (Monica and Tom Selleck was never about Tom Selleck’s character, just about Phoebe or Ross’s reaction to Monica and Tom Selleck, so therefore was really about Monica and Phoebe and Monica and Ross, yet again.
It couldn’t be about Monica and Tom Selleck, because that was so vile it couldn’t be funny. They didn’t fail to make Tom Selleck’s character a three-dimensional character; they didn’t DARE. And so rather than peripheral characters growing into their own roles, like Frasier Crane, or Lillith, or even Harry Anderson or Nick Tortelli, they just remained gags. Sick, disgusting, vile, decadent gags about how amoral and meaningless young adults were supposedly in the 1990s.
I’d also point out that Pheobe is a rip-off of Woody from Cheers and/or Rose from Golden Girls. But there was never any extra layer of humor, like Diane’s horror or Blanche’s baffled disbelief. And minus that, she comes off as ridiculous, ... which is amazing in comparison given that for crying out loud, Rose’s gym teacher was actually Adolf Hitler.