Ok. I don't think this is a big deal for the vast majority of blogs, like mine, that are hosted on GH pages. It's just HTML and some photos that are unique per post. But I also don't see why GH would put the number so low.
Because they have this one max-time for everything, from things that should be refetched frequently (html, unversioned scripts and stylesheets, etc.) to things that should be immutable (versioned scripts and stylesheets, images, etc.). They don’t understand your website. You do, and you can set exactly the right headers for the best user experience. Btw you can set the right headers with Netlify and Vercel as well.