Why 100% humidity is not water?

Because we are talking about the relative humidity of the air. When the air humidity is 100%, this means that the maximum possible amount of water vapor is in the air and the air cannot take more. In other words, the evaporation of water under these conditions is impossible. The lower the relative humidity of the air (that is, not 100%, but, say, 23%), the more water can pass into steam and the higher the rate of evaporation.