It's narcissism that's all it is. Same with when random person asks X model or Y Celeb to follow them, that moment of "elation" when the you see X or Y follow you and vice versa, X/Y are taking time out of their day to "follow" that person(s) and never care about it again.People wanna see their follower counts go up to make it look better to themselves and to others. More people will follow/go to an instagram page they see with 100k follower vs 1k followers given the same content, same everything.
Similar to Twitch or Youtube, if someone has a high viewer/sub count more and more people will likely go to them (GGX Gang TriHard 7 in chat )regardless of content vs a little known streamer or youtuber that has a low sub/viewer count but still churns out the same/similar content. Of course twitch/youtube and instagram all have variables, but again you put the same 2 things beside each other and one has a pretty little bow and one doesn't more than likely you'll grab the one with the pretty little bow.