Not to mention that a person with an active covid infection is less likely to feel like hiking than making a quick trip to the store.
You're forgetting that many people can have the virus and not show any symptoms, but still can infect other people.
So even if you're feeling fine, you may still be able to spread the virus unknowingly.
As for masks on the trail:
I've been pretty picky as to where I hike and bike.
I generally take a long bike ride in the mornings so I can avoid most people on the trails/sidewalks.