You don’t pay the cloud providers (just) for their computing power.
I feel that your comment is a red herring. Using someone else's computer does not mean taking advantage of computing power alone, and nothing more. In fact, the point of paying someone else to use their system was never computing power, specially nowadays.
Ease of scale? Ease of development cost? Or other marketing material?
AWS's shared responsibility model is quite clear in stating that end users are ultimately the ones responsible for security and compliance. So your points are moot.
"Flexibility" in this case is so vague that it's meaningless. I mean, is there anything more flexible than owning the entire infrastructure and software stack you chose all by yourself?