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" 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.

i don't know of any rf lens usage case

That can be changed. They are quite common for ku-band and up. Think satellite communications, radar, point-to-point links where you want high but can't use a dish, e.g. for weather reasons.

Look at this nice lens antenna:

Or these nice insets to convert an open waveguide to an actual antenna

There is the Luneburg Lens, which can be used for various applications, from optical to RF. Luneburg Lenses are generally spherical, made of concentric shells of material with a stepped refractive index for practical purposes, but ideally the refractive index should be continously varying.

(image from www.rfwireless-world.com)

Applications are radar reflectors, microwave antennas and laser collimation.