Okay, this is something I can work with.
It immediately invites a follow-up question:
Why do people want to play a cooperative game that expects teamwork...by playing whatever they want with zero consideration of their friends and what they're doing?
Because that, that very specific thing, is what I meant by asking for a triangle and then demanding that it have four sides. That is, you sign up for something that tells you, over and over, in no uncertain terms, that it's for teamwork. Meaning, the team should work together. You should be thinking about that. You should not be totally 100% self-focused with zero interest in or understanding of your friends and what they're doing/interested in.
But to "just play the characters they want", you are very specifically saying that the players completely ignore their so-called teammates. Each one focuses on herself and only herself. Each one selfishly pursues his own gains and benefits, without any thought to cooperation.
Those two things are incompatible demands. If D&D is to be a team game--and by all indications, the overwhelming majority desires it to remain so!--it cannot also be a game designed to let players select their characters with significant (or even total) disregard for what their allies are doing.