Is it because Starbucks has the most efficient cost outlay for long term investment in a user's needs?
Or is it because people just want some damn coffee and there's one on every corner?
Or is it because, in a world full of places to buy coffee, one place gives you everything you could ever dream of in a coffee place?
That is what the big cloud providers are. They are Starbucks. They are not the cheapest. They aren't even the best. But they are everything you want.
If your company gets Starbucks-huge, you don't need to buy your coffee from Starbucks. You have your own deals with roasters and your own supply chains and baristas and coffee logistics experts.
That's why some companies build their own. Not because it's a better idea (it's not), or because Starbucks costs too much (compared to the investment in re-creating Starbucks?). It's because they are a business that effectively makes their own coffee already, so it makes no sense to pay Starbucks for it. Of course, they won't have a Starbucks once they make their own coffee, but they will have served their needs well enough.