Here's why:

* Sugar molecules are small and transparent. They don't have a distinct color of their own.

* Dissolving is a physical change, not a chemical one. The sugar molecules don't change their chemical structure, they just spread out and become surrounded by water molecules.

* The mixture becomes homogeneous. This means the sugar molecules are evenly distributed throughout the water, making the solution appear clear.

You can't see the sugar molecules individually, but you can still taste the sweetness and measure the sugar concentration in the water.