When you have a cast as large as The Big Bang Theory, giving everybody a raise means a lot more money, and at some point, you reach a level of diminishing returns.
A renewal for a Season 13 will mean new contracts for everybody involved and new contracts means raises.
The five main cast members already make something in the neighborhood of a million dollars per episode.
Regardless of what the show makes for CBS, at some point, it won’t be worth it.