>>110874imageboard advocates brought up on the received wisdom argument for imageboard culture tend to make a case like this:
on a forum, people know your personality, your post count, your membership of cliques and so on, so when you say something they won't reply based purely on the content of your post, your view, etc, but also taking into that account. if you're in with the mods they won't tell you that you're an idiot, and if you're unpopular they might not tell you that you're actually right. this can't happen with anonymous boards.
now that's true as it goes for the most part, but it's like making a wish with a genie, because what happens is that people - still the same tribal little monkeys they've always been - still feel the need for an in and an outgroup. when the forum put your registration date up and kept a record of you saying you used to post on furaffinity forums back in an off topic thread that was all well and good, but on 4chan that's not an option. so how do you distinguish between ingroup and outgroup? well, you've still got the content of the post, and the content of the post is a gigantic collection of shibboleths. reddit spacing. no capitalization. use of "y'all", use of "problematic" in any context ("a sugar based engine-oil may be problematic"), tumblr filename, normie/normalfag, doxing/doxxing, and that's before you even get into selecting arbitrary opinions.
so, in summary, because it is a shibboleth associated with people they don't like. but the underlying dynamic is more interesting on imageboards. someone on reddit can also notice that it's common on tumblr/twitter/etc to use "y'all" to replace "guys", but only on 4chan does it truly evolve to being a shibboleth because there's so little by way of alternatives.