What’s the reason for doing that? Can’t you host a static page for 5 dollars basically everywhere?

Even on expensive AWS it’s 5 dollars: https://aws.amazon.com/lightsail/pricing/

lightsail is basically the lowest tier of AWS offerings. It's designed to compete with providers like linode or digitalocean, not gcp or azure. Moreover, lightsail is IaaS - you get a vps that you have to set up and administer yourself. Squarespace/Netlify is PaaS or SaaS - all that is handled for you. Maybe you think all of that isn't too hard and isn't worth the $14 or whatever premium, but it's still comparing apples to oranges.

Any reason to go that route over just making an S3 bucket publicly accessible and pointing dns at it? Should cost roughly $0/mo unless you’re getting massive amounts of traffic.

Cloudflare pages has been excellent for my 11ty site. The free tier is great, prices after that are reasonable.

Also - there's a bunch of integrations with their other stuff (serverless, D1, etc.) You'll get more lock-in, but you can scale up the static site to become basically a full frontend-backend app from there.