They are just never assigned. I thought they were specifically set aside to be used in fiction, but it seems like that might have just been 555-01XX. But because of the association to that, no one ever wanted a 555 number for anything else and now they are all unused except for 1 number that leads to ‘directory assistance’

https://en.wikipedia.org/wiki/555_(telephone_number)

Good catch! I didn’t know that’s what it was called. Having just looked it up, 555 as an area code and as a prefix are both not in use in the US, other than for directory assistance.

555 was specifically set aside as a fake area code. It’s used a lot in cartoons/TV/movies and is known that it won’t work for anything.