Out of 275 ships on the Navy's "Battle Forces" list, fully 23% of the Navy's ships are Arleigh Burke-class "destroyers," and 25% are nuclear submarines. Throw in a few Ticonderoga-class cruisers (8% of the fleet) and 10 operational aircraft carriers (4%), and well over half the fleet is made up of just four types of warship.
Why doesn't the Navy have a more flexible toolkit, with more types of vessels designed to tackle more specific sorts of missions? In part, it's a question of cost. More ship designs mean more expense to maintain them. In addition, when the Navy has tried to introduce new ship types to the fleet, it's sometimes been shocked to find the ships arriving over budget and behind schedule.