That is just because most people leave when they don't like something. In a bad environment you would expect the majority of those who stay to still find the environment good and themselves rational, with only a few outspoken against it. It is a relatively basic concept in markets, sociology, biology, information systems and reasoning.
The person you and the comment next to me are talking about is not here to defend themselves. They are specifically not here to defend themselves because they aren't allowed to. The justification for them not being allowed to is because they talked badly about someone else. Yet, it is apparently accepted to talk badly about them.
Your are even saying that we shouldn't trust this person because they in your opnion engaged in ad hominem attacks. But by doing so you are arguably performing an ad hominem attack yourself. And while you can certainly make a meta argument against that it would be close enough for many not to touch it.
This is why many people leave. Not because it's the Internet. Most of us have been around that. But because it is bad version of it.
You might think this doesn't have real consequences so I will give you an example. Because of the environment on Hacker News there is a lack of successful Europeans. This in turn results in there being an overwhelming consensus that is less likely you make good amounts of money in Europe. But this often isn't the case if you know what to do. It is just that those who do either leave or stop talking about it. Which continues the existing narrative as the participants reality continues to match their opinions. While in reality their opinion may not really be supported.
An environment with one sided environment isn't harmless. It might actually give you a worse life. But the same argument as before also applies here. Those who appreciate this are the first to leave. Leaving those who stay blind to what is happening.