It has nothing to do with companies not trusting each other (although it's true they often don't). Companies don't use their competitors' services because they

It has nothing to do with companies not trusting each other (although it's true they often don't). Companies don't use their competitors' services because they're...competing. Not because they don't trust them. Often, if there is an area where one company isn't competing with another, they'll form a strategic relationship in which they "trust" each other to provide services to their respective customers. So not lack of trust, but rather competition is driving the phenomena you're describing.