I casually link my profile to my "identity" here because the discussions I'm involved in here carefully stick to things I'm happy to have linked to my name. That is partially because the culture here frowns upon a whole lot of types of discussions that would lead to comments I'd be unhappy to have come up when searching for my name.
To some extent of course that means people can afford to be ruder on Reddit than here. But for my part at least I've "invested" as much in my Reddit username as I have in my username here, probably more. Just in different aspects of my life. What I say on Reddit has less effect on my work life, but just as much or more on my social life (most people around me are not on HN; most of them are on Reddit), and so I'm not about to "burn" my Reddit account.
The greater difference here I think is the same as you see on smaller subreddits: A relatiely large proportion of the comments come from a small pool of heavily invested long term users.
It doesn't matter if you know who we are in real life as much as it matters that newcomers that don't settle in to the culture here will be quickly pushed away (often after a number of downvotes and "this isn't reddit" comments) by longtime users that outnumber the newcomers. That happens on many subreddits too. But when you go to the bigger ones, the turnover is huge and it becomes impossible to police without very dedicated moderator teams which many subreddits lack, and the trolls congregate where they can get away with their trolling.
There are plenty of subreddits where I'd be happy to go by my legal name. The reason I don't is more down to already having an established identity over the course of many years and no reason to.