> why isn't there an SSI equivalent in the HTML spec?
Because HTML Imports died in the spec drafting process. Unless you want a client-side SSI equivalent that doesn't require any JavaScript, HTML Imports wasn't that. I'm not aware of an alternative proposal that doesn't.
https://www.w3.org/TR/html-imports/
https://webmasters.stackexchange.com/questions/127482/on-wha...