Because any idiot can do it. They're not worth the same money as someone with REAL skills.
$15 isn't something you pay a skilled worker
that's more to the order of $25-$40, in the trades
and $50-$200+ for IT-related abilities
3-4 extra bucks would go a long way with a lot of people who've put in years with the company and paid their dues
but they're no sliding scale, it takes a promotion, basically, and there's only so many of those available
fuck Starbucks
ps. it's a lot more than "serving coffee", that's just one of many things they have to do
besides janitorial work, balancing the cash register, inventory, and other things