Urban boundaries make housing unaffordable

By increasing the value of land inside the boundary. From REASON:

In too many metropolitan areas, housing is no longer affordable for middle-class households, especially in markets subject to "urban containment," now the world's dominant planning regime. According to planning experts Arthur C. Nelson and Casey Dawkins, urban containment draws "a line around an urban area"; it includes urban growth boundaries and greenbelts. It is "explicitly designed to limit the development of land outside a defined urban area, while encouraging" infill, to limit or block organic urban expansion.

Urban containment is intended to increase urban land costs. Shifting demand inside the contained area produces an abrupt increase in land values at the boundary, distorting the land value gradient. As Nelson and Dawkins say, "This shift should decrease the value of land outside the boundary and the value of land inside the boundary" (emphasis added), which effectively sets a higher "floor value" for urban land. This is the "urban containment effect."