There are a lot of things to take into consideration that complicates things. Those ‘Red Wall’ Tory MPs know that they’ll mostly all lose their seats at the next election so are probably reticent about rocking the boat now and will just want to milk as much out of their position as possible now.
There is no rule to say a general election must be called after a PM resigns but it is convention that it happens soon afterwards so you can see the position those MPs face.
The leader of the opposition can also call a motion of no confidence that the government must schedule for debate in the commons, whereas if it comes from any other MP the government can shut it down. Starmer has that power but is not using so Labour are not confident they can win that vote at this stage, or maybe they are willing to play the waiting game knowing they’ll win if Boris is in power.
Which leads me to my last point, surely the cabinet and Tory hierarchy know that they can’t win a GE with Boris at the helm. So they need to do something about it, which is why I can’t see him lasting much longer. There is the slight possibility that they are willing to spend 5 years in the wilderness and hope to get back in 2029 after an overhaul of the party. It’s a tricky one I honestly don’t think they are Machiavellian enough to be thinking about the schematics of these things, they’re just dumb posh fucks who are scared of Boris. We shall see.