I can only really answer for me. To be honest you wouldn't know I avoid people unless you had got quite close to my nuclear family, I fully agree that those announcing it every 3 seconds on social media are attention seeking. In my case mostly people I work with assume I have a reasonable social life because professionally I am paid to put on a persona. I don't lie but people assume it mostly.
In truth there are several reasons. I grew up in a neuro divergent home , I have a neuro divergent son and I'm probably on the list somewhere. The "world" doesn't like people like us and after you've seen a few sneers you start to back away (although still working hard that DS doesn't need to retreat ). I also spent 20 years in a career that specifically showed me the worst of human behaviour. The nature of what I did for a long time specifically put me in those situations. It taught me that people ar enot in fact on the whole good.
Ironically it was usually the arrogant self involved and self appointed savers and helpers that hindered the professionals rather than the service users themselves. It taught me quickly to never trust certain groups of people.
I just learnt to trust very few and the older I get I see little point in pretending for the sake of it.
The key is partly in your post, I abhore people who think that I need to be pushed or encouraged to see others. The people who witter on that I really do want relationships with others and its ok not to be scared and any version of faux concern and "help" to get out there. I'm an adult I know what I want. I am not covering an internal need for other humans. I have my humans. I am done. I find it irritating to have twittery bluebirds squawking that I couldn't possibly be happy and that deep down I need interaction.
No. I . Don't. Frankly if you had met the people I had over the years you'd learn to be a bit more discerning about the people you spend time with as well.
Honestly , take two seconds to search MN threads. It clarifies that you will be judged on what you look like, whether you have bath mats, what shoes you wear, how you eat, how you parent, whether you use specific words, how quickly you reply to text messages, whether you send cards, whether you are educated , whether you earn a little or a lot, examples without even touching the surface. Yes, completely shocking not to trust people isn't it?