I recall hearing, a long time ago...
...that there's a relationship between the wavelength that the collectors pull in and the maximum effective separation of the collectors. That the smaller the wavelength, the less separation you could use and get an effective benefit. Radio waves are huge compared to lightwaves, and so you will see much larger spearations in radio arrays.
I get the feeling that I once understood the reason for this, but I can't remember it, and I don't seem to be able to re-derive it.