Adding numbers thereafter could mess up copyfitting and page breaks. Paper publishing was the durable medium of choice for centuries. Page number referencing was OK as that was all there was. When done, folio numbers are often at the end of the page, not before them. Paragraph numbering is finer-grain than page, but subject to ambiguity when done algorithmically.
Many electronic documents are dynamic.
No sequential paragraph numbering scheme would survive revisions, without gaps and sub-numbering injections. Reorderings would get original numbering out of order. An SGML way could assign a unique identifier to each tag. A means to have fractional numbers would allow injections. Missing numbers would indicate omitted materials.