How much would you pay for 2 months of infrastructure engineer time? And how many millions / tens of millions / hundreds of millions are you imagining being spent on overpriced AWS services?

(Also, those AWS services are not engineering-free. I tried to migrate a system to RDS once and gave up after quite a few hours when I got to the part of the documentation that suggested that I edit my sql dump using sed to get it into a form that RDS would accept. No, thanks.)

But unless you're on a PaaS, you have "infrastructure engineers" already. So why not at least let them make back their salary by making them built a cost-efficient infrastructure?

This is rarely actually true but it's a common falsehood told by people who have financial interest of keeping everyone on AWS.

And that includes engkneers that only know how to use AWS and are terrified at having to learn something else.