Why do humans need to wipe after they poop, when animals can just squat and plop? Why do humans need to shower or they'll get rashes and smell awful, but animals don't?

 We are genetically still wild hunter gatherers. Plus, today there is more advanced bacteria and viruses in the world.

The paleolithic man did not need what you describe above. If you have the diet of a prehistoric hunter gatherer, your diet will be such that the volume will be low and the fiber content will be high and hydration low. Early humans did not eat at Taco Bell. Evacuation of bowels would normally happen in the form of one small hard and relatively dry mass. Nearly clean and that little non-wiped amount would not bother the person. Kind of like animals are today. Animals don’t need to unless humans want to make them smell better. Otherwise , animals just stink and have fecal material matting their fur until it rubs off.

On rashes and smelling awful, that is a creation of clothing, prevalent new bacteria, and modern aesthetics. Early humans put up with strong odors because they were used to it and did not have the social stigmas we attach to them today. Even today, some societies connect strong body odor as a mark of the personal strength of an individual.

Original humans would overcome rashes and such by developing a strong immune system from absorbing plenty of wild bacteria as they grew up. They never wore diapers as babies or washed their hands.

Sure, they got rashes and sometimes got dysentery, but they grew resistant to it, and tolerated it or died. All you had to do was reproduce a few times and make it to 30 years of age. You could pretty well accomplish that even if you had a stinky rash.