Language is contextual, hanging out with your buddies, telling them the fuck off is perfectly fine. They can lean on your shared history, your colocated activities, and the relationship is stronger than just the language.
Online, everyone is a stranger, so the hostility in the communication is the entirety of the context. Moderators, famously, don't want hostile places that people don't want to be in. Allowing open hostility, just encourages drive by hate and brigading. Not a constructive dialogue, which is what most of us are actually here for. Human interaction.
So- fuck you, go to 4chan if you want to yell at people, and externalize your self hate.