You don't want bits of food stuck to the pan. Bits of food still stuck to the pan go bad. The oil that sticks as part of seasoning has much smaller molecules as

You don't want bits of food stuck to the pan. Bits of food still stuck to the pan go bad. The oil that sticks as part of seasoning has much smaller molecules as it polymerizes, so you don't have bits of stuff that goes bad. If it comes up with soap, it's not bonded to the pan.