Agoraphobia

The fear of being exposed, with no possibility of protection, shelter, or escape should something bad happen.

Agoraphobia is most strongly associated with a fear of open spaces and large crowds, but this is a bit misleading. The danger of being exposed with no shelter is sometimes the fear of being attacked, or left vulnerable during a catastrophe of some kind, but it is also often the fear of being visible to other people and being thought about and judged by them.

Going outside invites the possibility of making a mistake or committing some social gaff that will be seen, judged, and punished. For some people, that mistake is the fact that they exist in the first place.

Chaos and Control

On top of this, life inside of your own apartment/house/studio/office is controlled, and if not necessarily tidy or clean, it is untidy and unclean in a known and predictable way. The outside world, on the other hand, is loud and messy and chaotic in a totally unpredictable way. Other people, in particular large crowds, are frightening because human beings can be erratic and dangerous when things go wrong.

Ultimately, the common feeling to all experiences of agoraphobia is that the outside world is fundamentally unsafe.