I mean it sounds like you just don't like drinking that much. People like different things.
Personally, I love it - in particular, the fact that any night of the week you can have one or several unique, powerful sensory experiences to savor and enjoy. Inebriation is basically an unfortunate side effect, for me, if my body stopped metabolizing alcohol after my first drink I'd be perfectly happy - what I'm into is the tasty drinks themselves.
I can have a moscow mule, a mojito, a margarita, and an old fashioned, one after another, and they're all delicious, and all different, and I love that. I can go to another bar tomorrow night, have those same drinks, and yet they'll all be slightly different than the last. Variety is the spice of life!
It's like the difference between a snack drawer filled with your favorite snack, versus a snack drawer filled with a dozen different snacks - you can, on a whim, choose any particular flavour or form you happen to be in the mood for at that moment. That's absolutely delightful for a hedonist like me.
... of course, that's a lot of sugar and salt, which isn't particularly healthy, I know that. But in terms of questions like "Why would you ever have more than one drink?" or "Why would you drink often" or even "Why would you drink alone?" - that's my answer, because I like the way cocktails (and beer, and wine, and cider) taste; And one important factor to enjoying my life is doing things that I like. Simple as.