Same reason we try to pretend animals are really humans with fur coats. Humans have an unfortunate tendency to anthropomorphize everything, even inanimate objects, and part of that is the expectation that animals want the same things we want, such as the fame and money received by performing for an audience. Once we can pretend the animal enjoys performing, rather than having been trained to think it will starve or be killed if it doesn't carry out the motions the trainers have taught it, people find guilt-free entertainment in trained animal acts, and in some cases affirmation of their deeply held "humans in fur coats" beliefs.

For this, people will pay lots of money, which encourages more people to train animals as a career.