TouchWiz UI is pretty awful looking but that doesn't matter since you can just get a custom ROM.
Sure, with hours and hours and hours of time wasted learning how to root the phone and flash the ROM. And if you get something wrong, your phone is bricked and Samsung won't fix it.
Consumers don't want to waste time on propellerhead activities. They want something that works.
Galaxy S III is VERY flashy out of the box. I have one. I looked at it and thought "hmmmm, maybe they've finally fixed Android's serious usability issues." For a couple of weeks, all was well (other than the cartoonish UI). Then, the old Android slowdowns, freezes, crashes, lockups, and incompatibilities reappeared and got worse with time. Underneath all the propaganda, UI tweaks, and super hardware, it's still the crusty, musty Android junkpile we all know and "love."