Why a navy cannot solve every problem with larger ships

Why do navies not choose fewer, larger, more capable ships?

Because one hull can cover only one place; force design must balance individual capability against presence and total fleet capacity.

Even an exceptionally capable hull can occupy only one place at a time. A navy with too few ships may be unable to escort, patrol, deter, and respond in several regions even when each individual vessel is powerful. Navies therefore balance unit quality against presence and the number of simultaneous tasks the fleet can cover. Several less capable hulls may provide enough combined capability for likely missions. Force structure is a problem of cumulative effect and availability, as well as individual ship quality.