Why do sweat, urine and feces smell so bad?

They don't because evolution and "civilization" have conditioned us to detest these odors.

The purpose of sweat is to help our bodies cool down by releasing water, along with several other products that eventually become rancid due to bacterial decay. Additionally, fresh sweat contains hormones that provide our brains with a wealth of information about the person perspiring (subconsciously, of course).

Doctors used to taste urine for sweetness (= diabetes), among other reasons. Many animals use urine as territory markers, or as clues that they are receptive for mating, or that they are pregnant.

Both the smell and the excrement give insight into a person's overall health.

Even single-cell organisms can distinguish between "food" and "danger" thanks to the olphactory sense (= smell), which is the oldest sense possessed by all living things. We "know" that sweat, urine, and feces are not food, but a dung beetle or a bluebottle may not agree.

The primary cause of the disagreeable smells associated with sweat, urine, and feces is the presence of bacteria that degrade organic compounds and release volatile byproducts. These compounds include fatty acids, which are found in sweat, and urea and other substances found in urine and feces. Bacterial activity on these compounds results in foul-smelling gases, which contribute to the characteristic odors.