Lenses and Antennas are two means of manipulating EM waves. While antennas are predominantly used at RF / Microwave frequencies, Lenses are used for optical/IR regions of the spectrum.

I understand why antennas aren't used at optical/IR frequencies: an optimal antenna should have a length on the order of the wavelength. For a nm to um wavelength, the resulting antenna would be too small to be practically useful.

But I don't see a reson why lenses typically used for optical/IR frequencies aren't used in everyday life for RF/microwave. Apart from optical coatings, I see no fundamental reason. Lens's focal length is independent of frequency, and Air's index of refraction doesn't seem to be changing significantly with frequency.