>> Why can’t they make visible light with an antenna?
> In terms of normal radio antenna designs, you could, I think, but antennas need to be sized proportional to the wave length of the light.
There is also one more difference to mention. For antenna design in radio frequencies the metals are taken as perfect conductors—meaning that the fields inside the metal are zero. This assumption relies on the fact that electrons in the metal respond the electric field basically instantaneously. However, the optical frequencies are significantly higher, so the movement of the electron cloud in the metal is no longer instantaneous relative to the outside fields. As a result metals at frequencies of visible light are way less "metal-like" when looking from the perspective of RF antenna design, so you can't directly use the exact same approaches for designing antennas.
There is also one more difference to mention. For antenna design in radio frequencies the metals are taken as perfect conductors—meaning that the fields inside the metal are zero. This assumption relies on the fact that electrons in the metal respond the electric field basically instantaneously. However, the optical frequencies are significantly higher, so the movement of the electron cloud in the metal is no longer instantaneous relative to the outside fields. As a result metals at frequencies of visible light are way less "metal-like" when looking from the perspective of RF antenna design, so you can't directly use the exact same approaches for designing antennas.