Most of the time I don't acknowledge anyone. I don't get acknowledged either, that's the Norwegian way of being polite

Seriously, it's a numbers thing. When I commute no-one nods to anyone, it would be like waving to every car you pass or greeting every ped on the street. Ditto for riding in the woods on a busy Saturday. But if I'm out in the woods on a cold day when hardly anyone's about and I haven't seen anyone for, say, an hour, I'm more likely to exchange a friendly nod or smile with some-one.

And it's a group thing. On my road bike, especially in club gear, I'll nod to other roadies, especially in club gear. Roadies are such a small group here that it comes naturally to acknowledge each other. But it's not because of the quality of their gear, the woods are chock full of mtb riders with bikes worth much more.