First of all, not everyone is gonna agree with your assessments of those covers. And of course there are other exceptions since the 5th Dimension. But I think the answer is because the artist (or a producer) saw the show and thought it would be cool to do it, often on a whim or as filler. Why don't more of them resonate with folks and chart? It varies but a lot of them aren't great as songs, at least not out of the context of their shows.