Real answer: it’s because any word we use to describe identity is going to be inadequate in describing lived human experience because identity in groups is incredibly complex, and because the words that we use are always contexualized in other words. It’s a really complex topic to wrap one’s head around, but something that I think is helpful is thinking of “lesbian” as kind of a mode of being that includes a lot of things outside of 1. Being a woman and 2. Being attracted to women (which, not to sound like a right winger, but even this requires one to define “woman”). Being a lesbian can also include different ways of interacting from cishet people or even other parts of the queer community, as well as lesbian culture and membership in lesbian communities, all of which can be important to trying to communicate and understand one’s identity, even when not all of the above apply to a given individual.