We speak of people as "ruthless," but we don't speak of them as "ruthful" or as having "ruth." (Well, no one I know does this other than my brother Tony!) We also call people "inept," but do we call anyone "ept?"
It seems root words like "ruth" and "ept" must have been around first and that then suffixes and prefixes were attached, and we've kept the lengthened words but not the original roots.
Maybe I'm wrong. Feel free to correct me if I am, but if you can't or until someone does, will you help us add to our of "rootless words" by leaving others you know of in the comments section. (David gets credit for this idea.)
We would have included "unwieldy" but looked it up and "wieldy" is a word. I guess it's just not used as much as the other. Perhaps "wieldy" is on its way to being a lost root.