What? For the same reason we don't build hydro plants everywhere, they depend on local conditions and all best spots are already used. Iceland can access geothermal power in entire country so they did.
“All best spots are already used”, is not even close to true for the US. We have barely begun to explore and evaluate potential sites and I see this as a huge future growth industry
In most places, thermal conductivity of rock is very low.
"The conductive heat flux averages 0.1 MW/km2. These values are much higher near tectonic plate boundaries where the crust is thinner. They may be further augmented by combinations of fluid circulation, either through magma conduits, hot springs, hydrothermal circulation."