I can't speak for every Starbucks customer out there, but I always assumed the reason is that it's a status symbol.
To some degree, they want to be seen with the cup. They haven't tried the competitors, so they assume Starbucks must be very good to justify the price. If they really enjoyed coffee for coffee's sake, they'd probably be going to an independent shop for their drink.
I think Starbucks' coffee isn't terrible, but I definitely prefer to brew it myself or spend the same amount at a smaller cafe, preferably if they roast their own beans.