First, because that's how most people, measuring these things, define success. For example, the Penn&Teller clip the parent post linked explicitly treats cessation of attendance as failure, and uses this as its sole justification for claiming that AA "does not work". You're welcome to see this as evidence that P&T are full of bullshit.

Second, those people declare themselves to be successful, by their own lights. They might be wrong, but I suggest that our first approximation should be to believe them.

But the third and more-important reason is more complicated. I don't claim to be an expert on AA on any level, but with my very limited data, I'm starting to believe that a better analogy than yours is this: "this gym is awesome... people keep coming back to maintain their muscle tone and cardiovascular health!" And for me - as a non-drinker - the alcoholism part is just the tip of the iceberg.