Netlify provides a global cdn and administrative features targeting statically generated pages.

S3 is more generic in its goals so it doesn't provide either.

Heroku on the other hand can host server applications which Netlify cannot so it is really a different service. You could host an API on heroku and the react app on Netlify.

probably the best reason is: we have our CDN spread over 4 different cloud providers. AWS and Digital Ocean could both go down (completely - though we're in several different data centers for each) and it would keep on keepin' on.