[Swordmaker]

This article reiterates the 'rubble pile' explanation, but notes that one end is lower density than the other. That could be suggestive of water ice. My view is that all the Earth-crossers formerly were part of now-extinct comets.

Unfortunately for your theory, every comet we've analyzed or visited has shown essentially zero water. . . and is indistinguishable from any other asteroid except for orbit and electrical charge.

[Telepathic Intruder]

“Extinct comet” implies that all the ice has been sublimated. Some comets break up due to outgassing, and I assume some just become extinct and turn into small asteroids. Maybe some break up and then reform, leaving no ancient craters. It’s just interesting, whether it be wrong or right.