Maybe it is regional. We are in upstate NY and I would never wear my shoes in someone's house. I would imagine any place I have been the bottom of my shoes would be dirty, and we do deal with snow and sludge 7 months of the year...
Just habit as it has always been that way. We don't think twice about it. Everyone we interact with (except my in-laws) take their shoes off at the door. I have even had girlfriends bring their slippers.
Lived there for 15 years and this was our experience too. You don't ask people to take off their shoes, it's just automatic. You expect that you will remove your shoes whenever you enter someone's home, and yes it was common for people to carry slippers or fuzzy socks if they were coming over for dinner (many people just kept a pair in their car).
We just moved to NC over the summer and we are adjusting to the norm of shoe wearing indoors. We have racks and don't wear them in our own home, but it seems that almost everyone else does. I have just accepted that this is the norm here, although I admit it does bother me a bit.
I grew up in a home where shoes were forbidden (my mother is a clean freak), but DH's family are the type who take their shoes off only as they are getting into bed.