You're not switching cloud providers. Amazon's not going to suddenly decide to jack up rates for EC2 instances on you. So the extra complexity just isn't worth it.

There is a hypothetical "but what if we honestly actually really really do", but that's such a waste of engineering time when there are so many other problems to be solved that it's implausible. The only time multi-cloud makes sense is when you have to meet customers where they're at, and have resources in whichever cloud your customers are using. Or if you're running arbitrage between the clouds and are reselling compute.