You guys are all wrong about the credits.
"Also starring" does not mean the same thing necessarily as "regular." Nor does it necessarily equate to being done because the opening credits sequence is only so long.
Much of it has to do with how much each actor is paid, which is also tied to how many episodes they sign up to be in. Or how many episodes their overall deal requires them to be in. This season we had an actor on our show who had been recurring for two seasons but who we definitely wanted back this season, but who wanted to go on to do other stuff. So for some reason we signed him to a "10 out of 13" - meaning he would be in 10 of our 13 episodes. He's nowhere near being one of the regulars or being a central character on our show, but his character is important to the show so he's listed as a regular now. If he's not in all 10 of those episodes, he still has to be paid. All because we wanted to hold him for use this season and not lose him to another show.
No show is done the same way, and no show has the money or budget to pay all the actors on it to be included in the opening credits. Why is Michonne listed as one of the regulars even though she was newly introduced this season? Because the writers knew they'd use her (and Morrisey as the Governor) in every episode.
Or you have a situation like what eventually happened with Jessalyn Gilsig on Nip/Tuck, who was a recurring actor on the show for a few years and who was then upgraded to regular status, but was only credited as such for the episodes in which she appeared, unlike, say, Cirroc Lofton, who even though he was listed in the opening credits for every episode of DS9, he wasn't in every episode, but he still got paid.