But really, maybe it's because of our diet? Our manure probably contains too low a pH to be valuable enough to use for fertilizers. I think some cultures use human manure for fertilization though.
Most people eat too many animal by-products which don't decompose very easily. But there is a book about this very process called "The Humanure Handbook" if you wanted to read up about the possible use of it.
Due to disease carried by feces (human and animal.) Contaminated water is what has ruined many crops of vegetables that have sickened or killed people all over the country.