This is a basic question to which I should probably know the answer, but it never really occurred to me until I checked the stats to see the games reffed by Abdou Ndiaye, who led an informative postgame Q-and-A after a recent Washington Spirit game. What I noticed is that he rarely ever works with the same officials twice. I found one other game in which he worked with someone who was on his crew for the Spirit game, and that's about it. I did a quick check of an English ref's assignments, and it's the same story. Why is that? Do assignors not see any advantage to having familiarity within an officiating crew?
Rather, I think there are often lots of reasons not to keep people in the same crews. - First of all, if you have a referee who is moving up the ladder quickly but his crew isn't up-to-snuff, it doesn't make sense to bring them with him. Likewise, if one member is underperforming, do you demote the whole crew? And if an AR is much better than the rest of his crew, is he going to be limited by them and not able to transfer to a crew with whom he could find international success? - If every referee crew was silo'd off into their own gang, it might be harder to achieve consistency among the whole pool of referees as they never get differing perspectives. - Logistically, especially in a big country like the US, it might be difficult to keep crews together consistently. Referees usually travel all over, but ARs tend to stay relatively more local (on the scale of east coast vs west coast). Mix in any potential schedule conflicts and it could be a bigger problem too. - How are new ARs supposed to break into the system if everyone is in a crew? You could put them in with a crew of 2 other rookies, but I don't think that's ideal at all. Usually it's good to have at least one other experienced official on the crew to guide the rookies. For all these reasons, referees need to at least be ready to work with different sets of ARs. And the best way to do that is to make them used to working in mixed crews, instead of becoming accustomed to relying on dozens of matches of unique individual rapport. I agree with you that, on a singular match, a crew with tons of chemistry is better than one without. That's why there are standard crews on international matches, for instance. But in domestic leagues that's a lot harder to pull off, and big-picture it makes more sense to keep everybody mixed up.
I suppose I'm surprised there isn't a happy medium. For a couple of centers I checked, it looked like they worked with different ARs nearly every game. Would it make more sense to work with 8-10 different ARs instead of 18-20? Then by the time playoffs roll around (or Open Cup games), there will be some crews that have worked together a decent amount. Maybe AR Joe Smith is at PRO playoff level and has worked with center ref Jim Baker five times and center ref John Waters five times. If Baker isn't up to playoff level but Waters is, then Smith and Waters can be paired up and will already have some familiarity.