Having been on several P&O ships we found (OK niave) to our disappointment that the shows were exactly the same on all of them. Very good the first time even the second time .....but after that
Assume you are talking about the production shows... and yes, most lines have several that are repeated throughout the fleet.
The production of these shows is expensive so easier to spread that expense. Plus easier to train the entertainers necessary to be able to fill in for vacation times, etc.
The other entertainment can vary, although most sign contracts and sign contracts lengths and fly in to and around different ships.
When people formerly paid large sums of money for a cruise, the lines were able to afford to provide upscale and constantly changing entertainment.
Now that most cruises are aimed at the shopping mall and soccer mom demographics, the entertainment is furst "dumbed down" to appeal to the masses, then re-cycled over and over - very much like the re-runs that show endlessly on American television.
Plus easier to train the entertainers necessary to be able to fill in for vacation times, etc.
vaction time doesn't exsist for the dancers/singers. unless it's an emergency, then a dancer can leave because we can work them out, but singers - dream on.