There is a theory I read about a few years ago that asteroids formed only after planet collisions. It makes a lot more sense because a cloud of gas and molecular dust wouldn't condense into an asteroid. Its gravity wouldn't be enough until the cloud got bigger. But after forming a planet the material would be solid. That would also explain the irregular shapes of asteroids. If they condensed alone they'd all be spheres.
The ring fragments must collide now and then but they're not as fragile as a satellite.