As for marketing: its that, and the sheer ubiquity of the chain. Can anyone really claim that any of the processed junk that McDonalds and Burger King extrude tastes good? (Even their fries are mediocre, imo) Yuck, but they are multibillion dollar businesses. Starbucks, like McDonalds, is "comfortable" for a middle class clientel.
Actually, I've never claimed to be an arbiter of good taste - in fact, some wags claim I'm a philistine - but yeah, sometimes the processed junk at McDonald's and Burger King taste good to me - damn good. That's not to say that you don't have a point re: "comfort." But that ain't necessarily a bad thang (not that you meant it as such - or did you?).
As for coffee, I just don't care for Starbucks at all. I'd cross the street to avoid it, but there's one over there too. I like to make my coffee at home (plain ole, boring, Chock Full o' Nuts), black, no sugar, and strong enough to float an ox.