not the fans partying outside bit, but why would FIFA prevent two teams booking into the same hotel ? for sure its not directly comparable, but in the TdF lots of teams end up sharing hotels just because its logistically easier
In the TdF there are 23 teams, often descending on a fairly small town, with a massive entourage of press & media & fans following them. The hotels are actually booked by ASO to try and ensure fairness (they're graded and everyone gets a turn in the better and worse hotels I believe). It's fairly common for a town not to have anything like 23 hotels of the size/quality required. That's a far cry from two teams staying in the same major city.
I know that doesn't answer why FIFA wouldn't allow it (I don't think they'd have any say tbh), but TdF teams share because they have to, not because they want to.
They often stay in the same hotels at altitude camps as well - because top hotels on the top of top mountains are fairly rare.
Anyway, if the team aren't prepared for this (noise cancelling earbuds, white noise etc) then someone isn't doing their job