There's a 2013 article on why 30% of Michelin-star restaurants went with Nespresso, which seems to have consistency down: https://www.grubstreet.com/2013/03/nespresso-sold-at-micheli...
It is a bit more startling in the United States because it is such a large country. Imagine crossing a reasonable fraction of the globe and the food is still the same! The comfort of the familiar is an incredibly strong lure and I believe it happens in ways that are so broad we can hardly see we are within them, like a bug on the surface of a tidal wave. Food and drink homogeneity is only one of the ways, but is likely one of the more primal. People of a certain age, when given Kosher-for-Passover Coke (still made with sugar) are delighted by the return to what they grew up with.
In any case, Starbucks wants a customer who, perhaps having had their fill of experimental recipes and lunches at new places, can reach for a stimulus just like they had before. It might mean a lot in an airport terminal after nights on a mattress you've never slept on before.
I remember visiting NY and being fascinated by many of the options I've never seen in other countries.
And so, people want replicable coffee drinks. And coffee can be a fickle beast to brew. Given that, unlike McDonald's which can freeze and ship, you must go from bean to drink on premise in a matter of minutes, I could see a focus on trying to ferret out a way to make that expensive cup taste the same from Portland, Oregon to Portland, Maine.