The parts of our brain that reassure us in social situations are actually starting to become vestigial or atrophy so when i interact with human beings in person there are parts of my brain that interpret body language and they do it without our awareness. When we start communicating online that part of the brain shuts off just like a language that we don't practice our brain is like hey we don't need this thing anymore. Our ability to be reassured by body language and behavioral signals decreases as that part of our brain decreases which results in increased social anxiety.