They probably don't. It's all part of some stupid infantile game that all parties seem to play (which is almost certainly one of the reasons why I have close to zero interest in the whole charade).
On the contrary, I don't follow politics hardly at all, but I'm guessing because he knows Sunak's massive ego won't every allow him to step down as leader – he'd rather face a GE. Therefore, the default is to call it and lose.
So, Starmer is effectively asking for a GE now, not for Sunak to...