Socks tend to smell first because they absorb and hold the dead skin, oils and other secretions from the feet which would normally dissipate if the feet were not covered. A body doesn't only sweat to keep cool, it is also eliminating waste which accumulates in the socks. They hold the waste against the skin of the feet making the feet smell. The sock also provides the moist warm environment for fungal growth which can smell pretty bad. If someone remains barefoot most of the time, it is unlikely they will have much foot odor.