It used to be "No one got fired for buying IBM." Now it's AWS. Many CTOs are very risk averse, and having someone to blame is incredibly valuable to them. My company has a large multi-region set of datacenters, but we're using AWS for a lot of new projects, despite being able to deliver similar services with better performance and cost, on-prem.
Some of the reasons are no one wants to be seen using old tech; stuck being the last COBOL dev. So there's a big incentive for developers and their managers to be using the latest tech; it looks good on the devs resume and grants them job security in the field, and it makes their managers look like they're sharp and on top of new technology.
Also, recruiting people familiar with AWS services is often easier than finding someone versed in on-prem tech. And for companies in less than desirable locations, this helps remove a staffing issue since it can all be done remotely.
And when something goes wrong on-prem, the CTO is expected by the rest of the C-suite to fix the issue. When it's a reputable 3rd party, he can absolve both himself and his team of responsibility.