Good question.
Maybe it's because there is a level of control people have here that they don't have in real life conversations.
There are times I come here to read with no intention of posting. Imagine being at a party and squeezing yourself in to a circle of people having a conversation. You don't say anything, you just listen. And when you've heard enough, you just walk away wordlessly. That's creepy.
Other times I'll reply to a whole string of threads. Sometimes I'll post something without any interest in how people will respond. Or I'll respond to something half a day (or half a month) after it was posted.
It's the control.
Why do some people with SPD enter into rambling conversations with each other?
Because they have stuff to say?
I think it's just a matter of different temperaments. Some people are naturally more talkative than others. This distinction exists separately from whether a person is SPD or not. Some people are more reserved than others. Some like to discuss ideas or concepts but don't have a social outlet to do so.
It is said that schizoids live deep in their heads. Nowhere is it specified how much is actually going on in said heads. But I have yet to come across any posters or posts in this particular sub-forum that point toward a lack of insight or intelligence. Some schizoids keep their heads tightly locked. Some like to let it all spill out on occasion. For this purpose, Psychforums is more preferable than a crowded bus.
Is it because it's not 'real' communication, so we can pretend that everyone else out there doesn't exist - and we're really just talking to ourselves in the mirror?
Again, it's all about control. I can be having a conversation with you here and if I should suddenly and unexpectedly lose interest, which happens a lot, I can make you cease to exist with the click of a mouse. Until such time when I choose to resurrect you.
Other times I post just to express something, regardless if it needs a response.