They do stream from satellites; that's why it's called "satellite radio". That doesn't mean they can use as much spectrum as they want; they only get so much spectrum to broadcast in, so they can either have fewer higher-quality channels, or more lower-quality channels, or (what they did) a scheme where different channels can have different quality levels. But of course, they assume that most listeners can't tell the difference between good quality compression and poor quality (too-low bitrate) compression, and aim for more channels to satisfy more diverse tastes.

As an aside, SiriusXM isn't just broadcast from satellites; they also broadcast in cities from terrestrial towers, because in cities the reception from the satellites frequently isn't good (too many tall buildings and tunnels, too much multipath distortion, etc.). This doesn't change the spectrum allocation they're granted though.