Phoebe was never a real main character
I mean she never got much. She was always there and had plots but they never carried over to new episodes. But for three tines:
-David: he pops up in episodes here and there
-the triplets: this plot was only added because Lisa got pregnant. Making one of the rare plots that carry over to a new episode.
-Mike: the only real continuing Plot for Phoebe.
I know many people don't like Phoebe amd everything but her character was basically just used to get to point A to B sometimes or for a joke.
She never got the proper time the other characters did. Like Rachel got like 3 episodes of being on the hospital giving birth but Phoebe got one. Chandler and Monica got countless episodes of engagement, wedding planning and the actual wedding. Ross and Emily even got a handful of episodes for all that. Where Phoebe only got two maybe three.