I confess that I don't "get" it, either. Not only do I not understand paying to go to WDW and spending valuable time standing in a line to buy coffee, but I don't get paying for coffee when I can get it for "free" in my own hotel room (with my favorite-flavor dry creamer brought from home).
I tend to think that most Starbucks customers wouldn't know the difference in a blind taste test, and that they see buying coffee from Starbucks as a status symbol. In fact, if you go to any of our local Starbucks, you'll see a line of women all identically draped in what they believe to be the vestments of wealth and success in this corner of the country: in short, irrespective of their age or actual socioeconomic status, they're all wearing the same North Face jackets and Ugg boots (which are, inexplicably, still a thing here) and carrying designer handbags. At a law firm where I used to work years ago, a couple of the secretaries would secretly pour the breakroom coffee into paper cups from Starbucks, which they'd reuse as many times as they could and walk around carrying with them everywhere, to make it look like they were high rollers who sprung for fancy coffee every day.
On the other hand, if that's how other people choose to spend their vacation time or their money, more power to them... and at WDW, each person in line at Starbucks is one less person in line at the attraction I'm headed to next, so I dasn't judge them.