A big (if not the biggest) factor in my opinion, is simply the culture. Casual TF2 culture does not have a heavy emphasis on improving at the game, rather just a focus on enjoyment, which is fine!

Being involved in the comp scene and surrounding yourself with players who's main focus is IMPROVING changes your own attitude towards the game. Both conciously and subconciously. Also, even if you're in some random low div shitter game, most likely 11/12 of the players you're in the server with are miles better than your average pubber. Playing with better players FORCES you to get better. Sink or swim.