Why does the Theis equation for well flow assume the water-bearing layer is very big and goes on forever?

The Theis equation assumes the water-bearing layer, known as an aquifer, is infinite in extent primarily for mathematical tractability. This assumption simplifies the complex partial differential equation (PDE) that describes the transient, or time-dependent, flow of groundwater to a pumping well, allowing for an analytical solution. An analytical solution is a precise mathematical formula that provides an exact answer, rather than a numerical approximation. By assuming an infinite aquifer, the model postulates that the cone of depression—the localized lowering of the water table in an unconfined aquifer, ....