

A few things come to mind:
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People don’t seem to understand or care when a post is just to vent and the individual isn’t looking for feedback. Some people have a really hard time not chiming in with their take.
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It’s a low effort reply that only requires a surface level understanding of the situation (and that might even be generous) so it’s very tempting to people who are itching to chime in on everything.
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Some people feel Very Smart when they make that comment because it means (in their view) that they are gifted at getting to the heart of the matter and seeing things as they truly are, without bias. There is also zero consequence to them being wrong about this.
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Sometimes people get defensive when they see their own behaviors reflected through someone’s vent post and it’s now personal.
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It’s sometimes used as a general saying, like “You are the problem with this country nowadays”, again the commenter feels real fucking smug about their pattern recognition skills.
I’m sure sometimes OP is the problem but I’m not sure it’s possible to have enough information to make that kind of judgement.

There are a whole lot of assumptions in this reply lol.
I’m not offended, but it’s a good example of people not seeking to understand what they are replying to before they add their two cents.
I understand this is what happens on the internet. It’s not possible to communicate perfectly.