It all comes down to branding.
Starbucks is an extremely strong brand, and anything they can do to ensure that is more money for them. The reason Americans like dark roast coffees is primarily due to a Starbucks marketing campaign back in the 60s/70s (iirc). At that time, the artisan coffee industry didn't even exist yet, because artisan coffee trees had yet to be "discovered" The trees were known, of course, but when grown at the same altitudes you grow other coffees, they produce an extremely poor product. It wasn't until the discovery that those trees grew very high quality beans at high altitudes that the artisan coffee market formed.
So what Starbucks did was roast the hell out of the poor quality beans Americans had access to, burning out the less desirable, generally sour flavors, and invested a LOT of money in getting their consumers addicted to the dark, burned flavor instead. Which wasn't THAT hard, because flavor profiles generally go lightly roasted high quality beans > dark roasted low (or high) quality beans > lightly roasted low quality beans.
It's essentially how they started their brand, and it's what allowed them to expand so well.
But now, with a well established and expanding artisan coffee market, Starbucks is having a harder time selling their product. They expanded on the premise that they offered the best product, which was actually true before high quality beans were readily available, but it's no longer the case. So now they have to sell the Starbucks experience, and the more they can get you to identify with their brand, the better. It's why they've sunk so much money into their branding. People go to Dunkin Donuts because it's cheap and it has donuts. People go to Starbucks because there's the perception that they have better coffee. Which IS true, when you're comparing them to other big coffee chains.
But when you're comparing them to small beaneries and coffee shops, that's generally not
It's a big part of why Starbucks is focusing far less on urban areas now. Suburbia and rural areas are far less likely to have a decent population of coffee houses and beaneries than urban areas are, which vastly reduces their competition. Starbucks easily offers the best coffee in at least a five mile radius from my house (assuming I'm not making it at home, which is what I typically do). But if I'm in Princeton? Yeah, no, I'm going to go to Small World
for better coffee.
The Small World and Starbucks near Princeton campus are within a block of each other. Small World is on a side street, where Starbucks is on one of the main streets through the city, with far more foot traffic. Despite that, Small World is almost always at least twice as busy as Starbucks. Because they offer better coffee at a similar price point.