Why are facebook’s open graph meta tags different?

I was looking into Facebook’s OpenGraph meta tags (og:), and they follow this format:

<meta property="og:type" content="article" />

Whereas, for normal meta tags (in HTML5), we use this structure

<meta name="viewport", content="width=device-width,initial-scale=1">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">

Facebook uses a seemingly proprietory property attribute on the meta tag. Which is different from what I’ve seen everywhere else.

So, then I looked up W3C’s official spec, and this is what it states:

Exactly one of the name, http-equiv, and charset attributes must be specified.

If either name or http-equiv is specified, then the content attribute must also be specified. Otherwise, it must be omitted.

So, my question is, why is facebook forcing it’s own flavour of metadata, when
the name attribute would just as well do, and is what is defined in the spec?

Source: facebook

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