[cripplecreek]

I would think this is the case only because the primary way of detecting exoplanets at this time is via the gravitational effect they have on the star they're orbiting.

Agreed. We haven't even come close to beginning to scratch the surface and there are hundreds of billions of stars in our own galaxy. Also there's the fact that we aren't really even looking for moons which probably number many thousands around the exoplanets we know about. After all, there are more than 200 moons in our own solar system.

[ETL]

Also, the “habitable zone” doesn’t take into consideration those planets with sufficient internal self-producing heating processes (volcanic, etc). Planets starting around Earth’s mass generate their own heat via radioactivity within their core. There are lifeforms that exist on Earth entirely independent of the Sun’s energy/photosynthesis, around undersea volcanic vents, etc.