I begin every one of my games with greeting the ones already on the server, and whenever somebody joins, I say hello to them. It's not even a thing about coop games. My mother taught me that.
I see your point. But let's take your example now: those people reacted to you. They were willing to play with you.
Some people on KF2 just join our game, completely ignore us, not a chat message, not a heal, no nothing. Then they die and then leave without a word. I'm unwilling to call them anything but griefers, and it's sadly very common.
I'm fairly certain the problem about bad pub experiences don't lie (at least only) with me, because since I ahve been playing KF2 I also got about 40 people on my friendslist, who I actively play with and we are friends now. Those people who could comprehend basic interaction AND know what they are doing in the game.