My guess is it some combination of:
- Liability. Even if other lounges have self-serve alcohol, they still could be dealing with increased liability as a result and just choose to still offer it)
- The classiness of having bartenders who can make decent drinks. Yes, self-pour is nice, but I keep ending up making the same basic things I have at home. I enjoy going to the Centurion and having high quality cocktails, and it wouldn't shock me if Amex thinks their target market feels the same.
- Shrinkage. I have a feeling those self-pour lounges experience a decent number of people who walk away with a water bottle full of whiskey. It might not be a problem when your self-pour is Evan Williams/Cutty Sark as it's fairly cheap and not as many people would want that, but when you've got Jefferson, Bulleit, and a decent single malt, It becomes much more tempting to people.
- Crowds. Most self-pour lounges in the US that I've seen are rarely packed like a CL can be. When they are that busy, having the alcohol out in the open makes it much harder to catch a minor drinking it and makes it much easier for someone to grab a water bottle full of alcohol.
Like I said, probably not any one reason but a combination of reasons that Amex feels is best for it, whereas other lounge operators just have a slightly different calculus, Even if some of the above pertain to them (e.g. AA having self-pour at their FLs)