My reasons are similar to the other commenter's-- I don't like referring to myself as "a lesbian" or "a gay" as in noun form; I'd prefer to just describe my sexual orientation with an adjective, because it's just one part of me. Yet when people use the word "lesbian" they usually expect it to be used in noun form. I often use it in adjective form-- I refer to myself as "a lesbian woman"-- but sometimes people get pissed off about that. Whatever.

I think this is also related to the sort of homophobia I heard growing up. I often heard straight women describe gay men with an adjective and say, "Oh, they're men who just happen to be gay," and nothing was assumed about their character on the basis of sexual orientation-- but lesbian women, they would refer to as "lesbians" and claimed that female homosexuality is always associated with certain negative character traits (e.g. being predatory/mean/violent/aggressive/masculine/whatever else they thought lesbian women were like).

I think that having a separate word "lesbian" for homosexual women encourages these stereotypes and de-emphasizes the fact that we are just... women. We are just human beings. u/soundsituation put it well:

I think that when a descriptor takes noun form it's far more likely to be seen, and felt, as an identity category.

And I would add that identity categories often imply group character traits or cultural traits. That would be misinformation: the only requirement to being a gay woman is being female and homosexual, that's it. (Misunderstanding of this fact is part of why we end up with TRAs who call themselves gay/bisexual and aren't same-sex attracted at all!)

Finally, I have come to additionally distance myself from the word "lesbian" more and more because of the homophobic "political lesbians" concept perpetuated by some radfems. Saying that I'm a gay woman, rather than "a lesbian," emphasizes objective descriptive information about me, and it also emphasizes the fact that I don't participate in this type of subculture. I'm just boring old homosexual, that's it.

It's a great question by the way, I don't mind you asking at all. :) That was probably a longer answer than you were looking for, though, lol.