Because bubbles are a thing. If you rarely interact with neurotypical people you can start to see them as a kind of unicorn even if there are a lot around. Our perception of how likely things are, is shaped by the bubble we surround us with.
But also because the world sucks ass and people are getting more comfortable talking about their mental health. Two of the three coworkers I was close to at my job were on the same antidepressant as me. That’s not a “I only communicate with similarly fucked up people” thing, these are people I didn’t have a choice in meeting. Not that your comment isn’t accurate, I think it’s also just you’re more comfortable talking about your mental health with the friends and communities you’re close to.
You said it better. Everyone around me “looks normal”, and what the post is saying is actually bizarre for me to be amazed at. The poster may have been surrounded by people open about their mental health issues, whereas in my area mental health is still a stigma to a large extent. Most people I interact with are ostensibly normal, but I know there some out there who take medications for mental health but they would not be saying and showing it to everyone.