People who screen resumes tend to assume that you are unemployed because of one or more other undesirable traits you have, like a tendency to be irresponsible or violent and antisocial. This assumption is a mix of broken societal wisdom and an association fallacy. FWIW, there's another fallacy committed when they themselves become unemployed; everyone else unemployed is a antisocial lunatic, but they are a good person who has simply fallen on rough times -- total cognitive dissonance, but they don't notice, even when it is pointed out to them.
It is not necessarily true that a company that is employing this philosophy is necessarily cutting out the best from it's hiring pool; but the inverse isn't necessarily true either. My suspicion is that the mean quality of the unemployed applicants to a company might be slightly worse than the employed applicants, but not enough so to represent anything significant.
What I can more confidently say is true is that most companies, and for that matter most individuals, haven't thought about this at all. What we do right now is the default, and they haven't yet been fired for relying on it, nor have they been rewarded for not doing so. Until they are, nothing will change.