I think this can be more generalized as: why do some people eschew anonymity online? And a few plausible reasons come to mind:

• a convention carried over from the pre-Internet days to be honest and frank as one would be in-person
• having no prior experience with anonymity or a basis to expect anonymity to last
• they're already a real-life edgelord and so the in-person/online distinction is artificial, or have an IDGAF attitude to such distinctions

IMO, older people tend to have the first reason, having grown up with the Internet as a communication tool. Younger, post-2000 people might have the second reason, because from the events during their lifetime, privacy has eroded to the point it's almost mythical. Or that it's like the landed gentry, that you have to be highly privileged to afford to maintain anonymity.

I have no thoughts as to the prevalence of the third reason, but I'm reminded of a post I saw on Mastodon months ago, which went something like this: every village used to have the village idiot, but was mostly benign because everyone in town knew he was an idiot. One moron in every 5 or 10 thousand people is fine. But with the Internet, all the village idiots can network with each other, expanding their personal communities and hyping themselves up to do things they otherwise wouldn't have found support for.

Coming back to the question, in the context above, maybe online anonymity is a learned practice, meaning it has to be taught and isn't plainly natural. Nothing quite like the Internet has ever existed in human history, so what's "natural" may just not have caught up yet. That internet literacy and safety is a topic requiring instruction bolsters this thought.