They can be. But most are lower concentration and work by supporting microbial action which aids in carbohydrate exchange instead of heavy loading of nitrogen, phosphorus and potassium. Depends on a what they are using and how much. In poor soils in harsh growing environments organics alone aren't always sufficient or heavy applications of high urea content are required (not great for the waterways, to be sure). Ideally, you need to manage your soil carefully over a prolonged period - and even, then, it's not always possible if the environment is inhospitable. Like most things, the truth is complicated.