These massive constellations don't make financial sense if you have to throw away your rockets every time. That is, they could launch on another rocket, but they'd easily get beaten by SpaceX/Starlink.
Amazon is making enough money now that they don't have to make a lot of sense immediately. They may choose to speculate on a future economic opportunity with concern to missing out on that if they fail to put up the network. Their market dominance also means they can only spend so much of their profit on traditional acquisitions realistically, they're going to be increasingly restricted there.
They're also piling up immense amounts of cash, with not much to do with it other than begin kicking it back to shareholders (the same 'problem' Google, Microsoft, Apple and Facebook have had). Needless to say, Amazon clearly isn't eager to return it to shareholders via a dividend or buying back its stock. Five years from now? They'll be drowning in probably $40+ billion per year in operating profit. They can trivially afford whatever it costs to do the launches with someone other than BO, even if it's very expensive. It doesn't have to make financial sense today.
They're also piling up immense amounts of cash, with not much to do with it other than begin kicking it back to shareholders (the same 'problem' Google, Microsoft, Apple and Facebook have had). Needless to say, Amazon clearly isn't eager to return it to shareholders via a dividend or buying back its stock. Five years from now? They'll be drowning in probably $40+ billion per year in operating profit. They can trivially afford whatever it costs to do the launches with someone other than BO, even if it's very expensive. It doesn't have to make financial sense today.