If you call scabies an STD, you have to call lice, the flu, colds, etc. STD's, because they're all spread through touching somebody. I refuse to admit I got an STD from my stepbrother
Scabies is, as you said, "highly contagious". It's extremely hard to get rid of. You and everybody you've been near have to cover yourselves in goo totally overnight, and in the morning you have to burn your sheets and clothes and everything you've touched. Well, by "burn" I mean "wash". Same idea. But anyway, it was pretty awful.
Also, when you have them, they like the warm parts of your body. Which means you get ichy rashes in your armpits and between your legs.