The hands are in different positions. Riders switch from the tops to the hoods to the drops frequently - sometimes to the aero bars, flat part of the drops, and curve of the tops as well. But you're right if you meant that most of the ride is done seated.

I was actually afraid to switch to just a riser bar on the mtb because I didn't see how you could ride with your hands in the same position for a whole ride. Turns out I was worrying for no reason. I guess all the standing, sitting, and shifting position for technical stuff and cruising helps to vary things enough.