Because the term "outlet" has been used in relation to electrical circuits and wiring for a long time, it is called an outlet rather than an inlet. It is a well-established phrase in the field that both consumers and electricians are familiar with and understand.
In addition, it is important to keep in mind that the idea of electricity moving "in" or "out" of a circuit can be a little perplexing because the movement of electrical current is actually a little more complicated than just moving in one direction. Yet, in the field of electrical engineering, the term "outlet" has grown to be generally accepted and utilized, and it seems unlikely that this will change anytime soon.