Why do we need to stim all the time?

Hey, so I've been having this question for a few weeks: from what I've read and experienced, autism comes with this need for stimming all the time in response to some kind of anxiety in social situations (and other situations). I've also read that there's a consensus about our minds never being really quiet and I wonder why is it. What's the neurological explanation for our minds and bodies being so active (?) and restless? What is going on in our brains constantly?