If I were to guess, here are several reasons:
- radio signal power goes down with square of the distance, so closer sources are much stronger than far ones
- any (be it software or hardware) filtering is not perfect, so a strong enough signal will probably still drown the weak ones
- it may be tad more difficult to filter out a close source because you can't assume it radiates in parallel lines, which to some extent you can with far ones (like for most practical purposes here on Earth we assume that sun rays are parallel)
- strong signals close to the ground means lots of reflections, which means more strong signals coming from random directions
- it's probably cheaper to make an exclusion zone in the middle of a desert rather than shielding yourself and upgrading the tech even more to filter out all the surrounding RF noise