Starbucks is in the business of selling milk, mainly. That’s where they make the most money. If you’re going to serve someone a “coffee” drink with 30 oz. of dairy, the espresso has to be as black as tar or it won’t be tasted at all. This is the main reason Starbucks roasts the coffee as they do.

The second reason is cheap consistency: it’s harder to do a medium roast blend and get a reliable flavor profile than to just burn everything to ash.

The open secret is that Americans don’t drink the coffee forward drinks. They drink sugar syrup options.

I'm an American although not a big fan of Starbucks (I like to brew my coffee at home), but I definitely prefer dark roasted coffee and buy my beans that way.