Because it's a massive franchise. When you go into a Starbucks they want it to taste the same no matter where you get one. People are creatures of habit and it's easier to get someone to get the same thing than it is to try out local shops wherever they go because it might be bad or too different from what they normally prefer. It's one of the reasons fast food chains are / were very popular in the US. It's not about the quality, it's about the consistency

I've only had it black as I always take it, there is no coffee I've had that tasted more over-roasted so consistently. Its like they made a mistake the first time, and just kept going with it out of tradition. Its vile.