Sweat from eccrine glands is watery and thin. It tends to evaporate pretty quickly and doesn’t usually cause a smell. But your feet have about half a million of the body’s 2 to 5 million sweat glands, so there’s a higher concentration of sweat. Some researchers believe that this evolved back when we walked around barefoot all of the time, and the constant contact with the ground meant that our feet got hotter and needed more sweat glands to stay cool. The sweat just evaporated off bare feet.