Because VOAs, alcohols, and phenols produced by anaerobic bacteria and yeast are very toxic to the roots of plants. Very small quantities are phytotoxic, that is, they kill plants. Aerobic organisms consume VOAs and alcohols. Think of bacteria enjoying champagne or other alcohols, just like humans. Of course, alcohol has a toxic effect on aerobic bacteria at high concentration, just like humans, but eventually, the alcohol, VOAs and phenols will get eaten, and the phytotoxic effect will go away.
Phenols are hardest to get rid of, because they are more difficult to decompose.
But, given time, aeration and the growth of aerobic organisms, any amount of anaerobic by-product will disappear. Again, your nose is a good way to tell. If anaerobic conditions developed, make sure the material is aerobic for at least 2 weeks before you try to grow plants in it. Someone once asked about ammonia smell in compost and compost tea. Ammonia is produced when certain bacteria volatilize ammonium and nitrate.