Well, yes, it goes without saying that that's the real-world reason. The point is to try to concoct an in-universe reason why Starfleet would do things that way, so that it doesn't feel so artificial.
I can think of any number of reasons why Starfleet itself would opt for that approach. Certainly, groups used to working together are going to function much more efficiently than people still growing accustomed to one another. Beyond that, I'd say that Starfleet seems to be a lot like other organizations in that officials in supervisory positions deliberately select those individuals whom they know they can work well with and who will do a good job.
crew stayed together for several years (albeit with other assignments in between on-screen adventures) in large part because they worked so well together. The circumstances of their being together after TVH are different because it may be that no one else was willing to work with any of them. David R. George has an interesting take on this in his Crucible