From the weakest to most compelling reason ↓
I hear that phrase quite a bit. It reminds me of Peter Griffin, who fills kerosene in his pickup to take off and fly with his car like an airplane. Consider films that did not record any success, despite an incredibly good cast (like Avengers: Endgame, or Waterworld). Because a top cast is no guarantee of success. It has a significant impact on the success of a film- especially if the film itself proves successful. Like: If the film itself is really good.
If a film is successful AND had a top cast, the top cast had contributed a lot to its success – maybe even more than we can imagine. If a film is not successful, despite its top cast, something must have gone wrong. Maybe the film received not enough attention or love. Whatever it suffered from, its top cast could not fix it.