The US Navy has other, very high priorities to balance against cost - national security, global security, freedom, peace, trade, the lives of billions, the lives of its own personnel. The US military may the largest budget of any organization in the world.
At the same time, they invest in those mobile nuclear reactors only for specific needs: attack submarines, ballistic missile submarines, and aircraft supercarriers. They calculate that the benefit of effectively unlimited fuel supply is worth the cost for those ships. The submarines can stay submerged for months at a time and have extensive range, and the carriers can move through the ocean, displacing 100,000 tons of water, to anywhere on the globe without worrying about the logisitcs of the enormous amount of fossil fuel it would require.
At the same time, they invest in those mobile nuclear reactors only for specific needs: attack submarines, ballistic missile submarines, and aircraft supercarriers. They calculate that the benefit of effectively unlimited fuel supply is worth the cost for those ships. The submarines can stay submerged for months at a time and have extensive range, and the carriers can move through the ocean, displacing 100,000 tons of water, to anywhere on the globe without worrying about the logisitcs of the enormous amount of fossil fuel it would require.