Because the "lesbians" you're talking about are mostly-mythical creatures invented by the porn industry expressly for men to like watching, silly! Lesbian is a complex signifier, because it has to stand both for this lovely beast and for the many women who live every day with a lesbian identity, including women that many men find distasteful. It's cliché and only partially accurate to characterize it this way, but it makes some sense: the man-centered lesbian vs. the woman-centered. Generally, the lesbians in "real life" aren't interested in having men watch them, no matter how much pleasure they're giving or receiving. Which is why it's lucky for you that they invented that other kind.

I like unicorns, they're pretty.