I assume because Starbucks assumes, or realizes, that clueless tourists will prefer to go to Starbucks rather than the better Martines, Bonafide, and other speciality coffee roasters, and they can make #lotsamoney fleecing said tourists. Possibly a better question is why would you allow yourself to be fleeced?
Though I don't know if Martinez, Bonafide and the others offer the concoction you want. If that's the case, Starbucks have a monopoly on you and can charge whatever they like.