Why do warships spend so much time in refit and training?

Why do warships spend so much time in refit or training when merchant ships operate differently?

Warships combine more complex systems and a wider range of missions, so merchant-ship utilisation is a poor direct comparison.

A large merchant ship normally concentrates on one transport role, with simpler systems and substantial redundancy. A carrier or escort must support missions such as anti-submarine warfare, air defence, strike, presence, airborne early warning, and electronic warfare. The equipment needed for those roles increases maintenance complexity. Even rust or a minor breakdown attracts much more public scrutiny on a carrier than on most merchant ships, adding another pressure on upkeep.