2) We want to call out people who were wrong (huge thing on the internet is not ever being wrong; nothing is worse than changing your mind), but it's too hard to keep track of everyone you disagree with, and worse you can't as easily get away with calling out an individual as you can a group.
I don't think those are the reasons, but rather the negative side effects. I think the reason we stereotype is because historically is has been extremely advantageous, biologically/sociologically speaking, to be able to predict certains things based on someone's appearance or background.
1) We want to broadly dismiss a group of people.
2) We want to call out people who were wrong (huge thing on the internet is not ever being wrong; nothing is worse than changing your mind), but it's too hard to keep track of everyone you disagree with, and worse you can't as easily get away with calling out an individual as you can a group.