It's more about the fact that Starbucks uses fully automatic machines and less-skilled baristas to pull shots. With a super dark coffee, you can get consistent

It's more about the fact that Starbucks uses fully automatic machines and less-skilled baristas to pull shots. With a super dark coffee, you can get consistent results (consistently bad IMO). If they went for lighter roasts, there would be enormous variability day-to-day which is likely not a great experience for customers who do like a good juicy light-roast espresso.