I have pet house rabbits living in my house and getting on my bed in my experience they prefer harder areas with good traction for resting. Floors are better than beds, carpet is better than hardwood or tile.
I would expect that when you see these rabbits, they have all four feet under them. Ready to bolt.
Being under or near something, provides cover from birds.
Being on solid ground with good traction, make a rapid departure easier
The gravel is going to make noise if a predator (or person) is approaching.
All of this can grant an extra second of escape time, for a dozing rabbit. The difference can be life or death.
One Pair of Rabbits can become 1300 in a year, the fact that the world is not covered in rabbits, is testament to how often they are preyed upon.
Additionally rabbits are crepuscular, being mostly active in the twilight hours of dawn and dusk. This is when they do most of their eating. Other times they rest.