As a dev I see multiple reasons that I've mentioned earlier so I'm not repeating myself here

But besides those that I already mentioned, looking just at the number of staff is a big trap
tons of projects fall into. Like, we have entire studies, laws and books about this stuff. You need to look at which level that staff is, you need to have an idea of the skill set that comes with your staff (character designers vs scenery designers), you have things like increasing salaries which might mean that a dev gets too expensive to work on a certain project (happened to me for instance), just overal interests of your staff etc. etc. etc.
Just to give an example here, this is really not that simple as you describe it here. If you want to have some form of continuous support, you can't switch up teams as easily as you describe it. Depending on the complexity of a project it can take 3 months before a new developer reaches an output of 80%. And depending on your hires (most of the time the hires you have are junior devs, medior devs are hard to find and senior devs are extremely hard to find), some of them won't ever reach an output of 80% in the first few years. I've seen plenty of projects get severely delayed or go massively over budget by switching up teams the way you describe it,simply because of how complex such things become in practice.
In the end, decisions regarding support are never simple and are influenced by a multitude of parameters, many of which we don't know anything about and the ones (beside profit) we have an idea of we have absolutely no insight on. So I often see people say "I see no reason", but that doesn't mean there isn't a reason, it simply means we oftentimes can't think of the reason(s).
(I really don't want to come across as hostile or know-it-all here, I sincerely apologize if I do, I've went back and forth on a lot of this and rephrased a lot, I'm just weighing in from the perspective I have.)