Well, I do that, so I guess I'll chime in.
First and foremost is the idea of not tying your site's build process and hosting to a company that could go under. Yes, Amazon could, but if I had to bet Netlify will either cease to exist, be bought out, have its free offering discontinued/changed/etc. long before that.
Second is that sometimes your site's build process is more complicated than what Netlify provides. For example, my site is a static Hugo site, which Netlify supports, but there's one crucial step in the build process where I turn my "resume.html" page into "resume.pdf" -- I have to run a Docker image that starts headless Chrome in order to render the PDF properly. As far as I can tell, that can't be rigged up in Netlify.
Finally, I was able to learn a lot about AWS by hosting my site this way -- setting up a Lambda to listen for GitHub commits, using the AWS API to launch an EC2 instance to build my site, how to configure S3 and CloudFront to serve up content properly, etc. And at the end of the day I get a site that I have complete control over and is approximately free to run.