Why is water wet ?

Water isn't wet. Wetness is a description of our experience of water;

what happens to us when we come into contact with water in such a way that it impinges on our state of being.

(source:google)

We, or our possessions, 'get wet'. A less impinging sense experience of water is that it is cold or warm, while visual experience tells us that it is green or blue or muddy or fast-flowing.

We learn by experience that a sensation of wetness is associated with water: 'there must be a leak/I must have sat in something.'