They don't taste nearly as good after a while, since the onion flavour comes from sulfur in the soil which it utilizes to make itself too pungent for (most) herbivores to eat.
Eventually, in the same soil, the onion stalks will get milder and milder, until there's no more sulfur in the soil. Onions (not just green) grown in soil entirely absent of sulfur will be sweet (esp if the cultivar was sweet to begin with)