the problem is that when people sign up to play competitive tf2, they arent signing up to be spied on. you would arguably drive away more players by having people be constantly monitored by league admins than by just letting the small subset of toxic players be toxic. you also empower said toxic people by making them feel victimized, as a result only making the problem worse because people will see overreach and become sympathetic to those players. also some people just think shit like that is funny, you may disagree, and you have perfectly good reason to do so, its just how some people are.
you then run into the problem of people taking screenshots and sending them to admins out of context, and this turning into a weapon to take out players you dont like.
got a match against a team with a crazy sniper next week? well lucky for you, he just made a joke in extremely poor taste in a discord with him and 2 close friends that you happened to be lurking in, now you have a free win!
it would be too much to deal with for an admin team that is already volunteer to begin with, so then there is more time spent dealing with trying to figure out if a discord screenshot is edited or fake than time trying to find what player is cheating or trying to help a new team leader figure out what the fuck they’re doing.
its a waste of effort and resources to police this kind of thing, its not like these people are playing for a brand for the most part, the reason big players in other eSports are harshly criticized is because there is a lot of money behind them, that is very much not the case with tf2.
tl;dr policing private shit makes toxic people more sympathetic to some people, spreads toxicity, too much shit for admin team to deal with