Every one of these large enterprises, including the cloud providers themselves, like stability. So they sign multi-year multi-million dollar contracts that expect a minimum spending and offers a negotiated discounted price.
It’s also not just the cloud, enterprises know they get the best pricing when they commit to a vendor and the vendor also bends over backwards to accommodate these large companies. As long as there are 2-3 cloud providers big enterprises will be just fine.
It's a huge project but there's not a lot of actual redesigning.