Why do Copper and Chromium only have 1 electron in their S orbitals?

The reason Copper and Chromium have a slighly odd electron configuration is because of stability. By only having 1 electron in the 4s oribtal, Chromium is able to have 1 electron in each of it's 3d orbitals, this configuration is more stable than by having [Ar]4S²3d⁴. Similarly Copper is able to fill each of it's 3d orbitals with 2 electrons which is more stable than by having [Ar]4S²3d⁹. Anything in nature will tend towards the most stable state, if any of the less stable electron configurations are created then they will be more reactive and as such over time only the more stable configuration will remain.