I grew up in a poor country. One Christmas Season, I took a part-time job at a bank's back office. The full-time employees would go to the McD across the street from the office building, and get a take-away. The journey cut their 60-minute lunch break by half. I remarked how expensive it was, but my colleague told me: "Remember how much they're making, they can probably afford it!"
:-)
People buy the expensive branded clothes because...they know what they are getting. No one really wants to walk around with a swoosh on their chest! Going for no-name clothing, there is no reputation to go by, how can you tell if that shirt is going to feel good or wear well? Muji does no-name well, you can trust their quality even if they don't put their name everywhere, but even that is a "brand."
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.