Answer: Because of your socks.
Explanation: Our feet sweat because we're wearing socks & shoes. It like wearing a winter coat. You'll usually wear socks because your feet gets cold. So if it gets hot, or you wear your socks for a long time and never take them off, your feet may get sweaty. Because your feet are getting too hot, so in order for it to cool, it will start to sweat, that's why when you take off your socks, your socks & feet are wet.