Reiterating and clarifying: objectively yes these players are not good. What they are doing is outperforming their ratings due to their usage, and over the long term they will likely turn into pumpkins. In a playoff series against a top tier opponent, they'll likely get blown out.
IMO the key is to see what ratings can be exploited with usage. If you're in any kind of competitive world it's hard to just buy up all the best players you can find. You score small wins along the edges of the roster, there are 25 players on it and your last three or four players are going to be ones it's nice to have outperforming themselves.
A recent small win I had was finding a Rule 5 outfielder buried in AA by his old team. Absolutely nothing special, minor league ratings across the board except he was a lefty bat with 95 vsR. I got three seasons out of him as a backup outfielder hitting .260-.270 and cut him when he got to arbitration.