These are great questions but it often boils down to a few factors. One is do they have the money put aside yet for a new project. Especially larger companies funding has to be secured per project, which means pitching an idea potentially doing a prototype etc.

The other is you generally don't need as much staff in the pre production stage of a project. Your trying to figure out what technologies you'll use, what style of art etc. Once you move into full blown production then you end up ramping up on developers, artists, etc.

And that's if you have an idea ready to go.

Basically to maintain everyone or even a large number of them you'd have to have multiple projects on the go to be able to shift everyone around.

Some studios handle this better then others. And I'm sure there are some higher level executives who don't really care. It's beneficial for some of them to save the studio the money between projects I'm sure that reflects in thier bonuses, which sucks.