Luca, I think we actually agree with each other, because I do consider each purpose-build service provider cloud a "proprietary architecture" too. Because it really is a different LEGO house from one to another.
Just as you noted, KVM requires certain special skills from IT admins. Nutanix is reasonably successful with AHV because they abstract you from these complexities, providing a solution "for everyone" without requiring the existing linux ecosystem and skills in-house. And service providers simply have those skills in-house already anyway - so KVM allows them to even better leverage their existing expertise to save even more money. And considering their business is about saving pennies (and fractions of pennies), no doubt they are excited to get to use something else that is "free".