Their recent troubles have all been with ships built by Fincantieri, an Italian shipyard that's one of the largest in the world. Don't see others of that brand having regular problems, so it seems a maintenance issue. Carnival could also be cheaping out on outfitting their hulls. Or both.
I say, rebuild the Titanic, oh wait, someone is already planning that
But yeah, what's cheaper, taking the ship out of service in order to do maintenance work or let them become cesspits or potential floating deathtraps.