Not sure the genuine reason but if I had to guess, nobody wants a sweat goin nuclear in their lobby and stopping the more dogshit players from having fun. Imagine dropping a top ranked league player against just about any team of assorted low ranks and the game is basically decided on the spot completely ruining the fun for them, but that top rank player is having a fucking blast. Many people prefer not to bother even touch competitive either because it's well... competitive.