Because boiling isn't evaporating. Water boils at 100 C but can evaporate at any temperature above 0 C.

The temperature of some amount of water is basically the average kinetic energy of its molecules. Some molecules will have more energy and some less.

Some of the molecules with more energy will be at the surface.

And some of those high-energy surface molecules will have enough energy to overcome the surface tension of the water and pressure of the air above it and shoot off into the air.

This is evaporation.

So, over time, any body of non-frozen water will slowly evaporate.