I can't think of anywhere I'd want to explore on a bike locally on road - I've driven literally every inch of road within a 50 mile radius
Hmm.. main roads, maybe, and possibly B roads.. but there are a hell of a lot of lanes in this country. I've ridden every lane within maybe a 5 mile radius of my house, if you count the countryside half. On the other side it's city so I have only ridden maybe 30% of the roads, if that.
Why do I like road? Well, I like being able to keep moving at a good pace for hours on end, and being able to cover lots of ground. You can see a lot of road and a lot of different places as you zip through on your road bike. A lot of MTBing is going around and around in a bit of forest, or based around a particular mountain, valley or some other feature.
I'd still choose MTB though if I had to, but I'd be sad about it. Fortunately I don't have to.