Lots of chain keeps the anchor pulling parallel to the bottom, to get it to hook up. Rope just lofts up at an angle and pulls the flukes up with it.

The idea of a Danforth or Bruce is to auger DOWN into the bottom, and bury itself. If the flukes aren't running parallel to the bottom while you're trying to hook up, it will just skate across the bottom.

I can see your point about gravel or egg-size rock bottom -- I guess in this case you just need mass.