I have seen multiple examples of sweepstakes run in the US, where entrants have to do something to enter the sweepstakes and the sponsor gets some benefit (e.g. long mailing list, etc.), with Official Rules saying that the names of winners will be posted to a specific Web site, or available on request, but then they just aren't. It's not clear whether there is actually any winner selected or not. (Even when names are published, like "Jane D. from Minnesota," they're often unverifiably vague).
It seems that no individual, not even an entrant, would have standing to take action because nobody can prove they specifically would have won, nor can anybody prove that there was no winner, because of the sponsor's ability to limit access to information. It also seems like there is no regulator whose job it is to follow up on those things, who actually cares (because the harms to any individual are relatively low, and there are higher-priority issues).
At first glance, running a sweepstakes with no actual winner seems unethical, but from a practical perspective, what if any negative legal consequences could such a sponsor actually face for doing so, and how would that practically come about?