I didn't start drinking until I was 23. I know, blasphemy and how the hell did I go from not drinking to brewing my own beer? Well, up until that point growing up in rural PA, I had only had what I now refer to as american piss beer (APB) and rum. Turns out, I have a strong disliking for both. So I was convinced I just didn't like booze until I moved out to California and was introduced to good beer, good vodka and good wine. Then it was straight down hill.
My taste, personally, as this is all we're talking about here, is that APB doesn't have any, and what little there is just isn't very good. Couple that with the huge amounts of carbonation (which I'm not a fan of and one of the reasons I don't drink much soda.) I like to take small drinks of something and savor the flavor and not have it turn into a frothy mass of foam in my mouth, which APB fails on both.
I can see how many people, even those who do enjoy many different beers (at which point 100% of them will have more body and flavor) which is fine. I've even been to wedding receptions in Phoenix in the middle of summer and still gone for the water instead of the Bud. It's just not for me (un?)fortunately.
I do agree that brewing an american lager with no major flaws is very very hard. The DMS and the diacetyl is quite difficult to get down to controllable levels for us homebrewers, and the big guys have come up with some very interesting industrial scale techniques to help with that. Which is impressive and testament to the quantity they deal with and the amount of time they've been doing it.
I also believe that most americans would in fact like different beers if they would venture out and try it. But given the marketing strategies of the big guys (and they're very good at it) and their control and influence with the distributors, people just don't try. The APBs are right there BAM in your face and people just go for the old standard, which people are prone to do.