I think that imaginary friends are partially created by kids through the feelings they won’t let themselves have.
Bloo for example; he’s selfish, right? Really selfish. But Mac is almost never selfish. Bloo wants things so Mac doesn’t have to.
And Bendy, his boy Gregory seems to have strict parents. Bendy causes trouble because Gregory wanted to but never would. He did what Gregory couldn’t.
Eduardo was weak so Nina could become strong. And Wilt was apologetic and anxious so that Jordan could be focused and confident.