Here's why:

* Proteins are made up of amino acids, which contain nitrogen.

* When proteins are broken down for energy or other cellular processes, the nitrogen-containing amino groups are removed.

* This process results in the production of ammonia (NH3), which is toxic to the body.

* Different organisms have evolved different mechanisms to convert ammonia into less toxic forms for excretion.

* Mammals and some amphibians convert ammonia into urea, which is less toxic and can be excreted in urine.

* Fish excrete ammonia directly into the water.

* Birds and reptiles excrete uric acid, which is a solid and can be excreted with minimal water loss.

So, nitrogenous waste is a direct consequence of the breakdown of proteins in the body.