@pointlessone@status.pointless.one short answer: oEmbed requires JS for publishers and consumers.
Longer:
For me, "simple" means a little HTML is all you need. I was inspired to come up with something that anyone could create with (existing) HTML text files. Zero imperative code.
Simple HTML also means easier / better support for static file hosting solutions.
E.g. if your website is on GitHub Pages or other static site hosting, you can’t implement an oEmbed endpoint (certainly not without an external service, which has other problems).
Whereas plain old semantic HTML (POSH¹) works great on GitHub Pages or other static site hosting.
Would be interesting to see if someone can build an oEmbed "shim" service that did discovery on rel=embed and then translated h-entry into oEmbed’s specific JSON vocabulary.
That way publishers would have less work to do (with plain HTML) and existing oEmbed consumers would support embedding more publishers.
¹ https://microformats.org/wiki/posh