A lot of it is this.

I also think that it's because a lot of the time these internet liberals looking for 'civilised debate' only understand it in the most penned-in, theory and evidence free, reddit way imaginable. They're deeply uncomfortable (and sometimes actually incapable) of engaging with arguement from outside that narrow frame.

I occassionally meet them on their internet debate style (mostly because other non-Hexbear users do sometimes read these threads when the OP goes back and complains on other instances) like in this recent thread example. And when you do and neutralise their main out by being actively civil at least one of three things usually happens, often in order:

• They take the presenting of an actually contrary view or arguement (often backed up with evidence) to their own as aggression because emotionally it makes them feel undermined when they're not able to reply with the same degree of confidence or demonstratable examples.
• They double-down on their own incivility and attacks in order to try and lure you back onto ground they feel more confident on; insult slinging and complaining about that
• They stop responding.

The fact is that most of the people who barge into our little commie/anarchist bar here demanding "debate" only recognise it as the second-hand snide one-upmanship of American liberals talking about marginly different shades of US party politics downstream of pretty facile liberal US politics media like PodSaveAmerica/Majority Report/CenterCast etc (I made that last one up). Being confronted with actual robust critique from an actually alternative perspective, no matter how civil and Reddit-rules-like it is, is clearly profoundly uncomfortable for them, and they react badly before going back to the likes of Lemmy.World to complain, call us all crazy (or even more ablist assertions) or paid trolls, and feel like they're on familiar ground again.