Technically a hot pepper is a fruit and fruits are meant to be eaten but no animal would eat something that's so painful to eat plain so why do hot peppers continue to be so hot?

Answer 1

Many plants have evolved to protect themselves from animals or insects that might damage or eat their fruits. Some use thorns, others bitter flavor, some have tiny hair-like skins, some produce poisons, and some, like hot peppers, have pungent oils that keep mammals and insects from eating them due to the odors or the spicy hot sensation they cause. The capsaicin in hot peppers (chiles) also is believed to protect them from fungi and not just from herbivores.

The sensation of burning is caused by capsaicin binding to receptors in the mammals that send signals to the brain indicating the sensation of a burn, however there really is no burn or tissue damage that occurs, it just feels like it from the brain interpretation of the stimulation at the receptors.

Interestingly, birds love to eat chiles and their seeds, and they are not affected by the burn of the capsaicin. So, like other fruits that depend on different animal species to distribute their seeds by eating and expelling the seeds whole, chiles can also be spread by way of birds consuming the fruit.

Now, to your statement, "no animal would eat something that's so painful to eat"...there are many animals (people) who love and do eat these very hot (including the hottest) peppers. We are known as "Chileheads". I love them and grow and seek the hottest varieties. There is a saying, "One man's poison is another man's pleasure", which seems to hold true in the case of capsicum (chiles).

Additional note:

If you bite off more than you can chew of hot chiles, try eating a banana, it is a fast and effective reliever of the mouth pain in people who are not used to eating chiles.