I rather suspect that at least some of the streamers were approached by representatives of Square Enix. Let's not forget, they're entertainers and the biggest have agents and contracts. That doesn't mean that they don't genuinely enjoy what they do or what they play...but to at least some degree the game's success is due to heavy marketing on many fronts.

Which means that Square Enix knew that the game would get more players but did not necessarily future proof the game accordingly. Even before Covid, server instability has been a common complaint in this game. Alas...