The main reason for the continued use of the phrase "gay and lesbian" is, quite simply, the existence of lesbians. In our society, "gay" means almost exclusively male homosexuals. Many activist lesbians dislike being called gay as they feel it deprives them of their separate, unique lesbian identity.
However, there are many opinions on this. There are lesbians who are fine with being called gay, and there are lesbians who will smack you pretty hard.
It's not a conspiracy, and it's not a subtle linguistic quirk.
It's politics, just like everything else.
Also, the dictionary definitions of gay and lesbian are never to be trusted, anymore than you would trust the dictionary definition of "rave".
This is a response to a soon-to-be deleted writeup, which asked why people use the phrase "gay and lesbian". The writer viewed lesbianism as a subset of gayism, and thought the phrase was ridiculously redundant.