Why don't we use lenses for RF?
In optical engineering, the choice between lenses and mirrors often comes down to aperture diameter: less than a few inches and lenses can be made cheaply and with high accuracy. Larger and costs increase exponentially, so even 6&quot; diameter systems usually work better reflective.
At RF frequencies, a 6 inch lens is on the order of a wavelength, and so not useful for focusing. It isn't until you get towards the edge of the microwave spectrum that the wavelength gets short enough for lenses to start to become practical.
Of course if you don't care about cost, and you don't mind it being extremely heavy, you could build a lens to use with a WiFi antenna. It just doesn't make much practical sense.