It makes sense that cloud companies are inclined to keep customers from giving money to competitors, but they way they sell it and structure services, reserved instances, and enterprise discounts is such that you basically are putting all of your eggs in one basket.
One of the selling points of cloud providers is managed services like SQS. If you run a multi-cloud architecture, you either can't use managed services, or have to build abstraction layers on top of them (and only use the features that exist in both cloud providers' versions of the managed service).
If you want to use a managed service that only exists on AWS, then that's obviously incompatible with a fully multi-cloud architecture.
And this is, of course, why they do everything they can do discourage it. Because if you do that, not only are you not reliant on them for availability, you can switch more of your business to the other provider(s) based on current pricing, and they do not want that big time.
If you make the engineering decision to go multicloud for whatever reason those are inherent trade offs you need to be aware of. They have their own agenda of course in addition to any actual fundamental "real" in bulk efficiencies that price reflects.
Nobody got fired for buying IBM, until they did.