There used to be one in Salt Lake City, but I think it hasn't run for a number of years. If you check pouet.net you can see what's around. Back in the mid-late 90s there used to be a fairly vibrant local scene on the East Coast of the U.S. and up into Canada, but it seems to have kind of calmed down and spread out. Lots of very small parties back in those days.

I imagine there's still lots of small parties in the U.S., but they just aren't as advertised?

The U.S. is just too spread out I guess.

Historically, no, there is no active demo scene in the United States. I was one of the few demo scene programmers in the US during the 1980's - 1990's, but it's never really been a thing over here.