maybe her existence or her essence
i think it's just as good an explanation for it as any hahaha, it's not like the fabula nova series prides itself on accurate plot-lines or logical thought

Etro - The Final Fantasy Wiki has more Final Fantasy information than Cid could research

this also doesn't explain it but perhaps etro just has no power for creation of other immortal beings. the wiki says she had no power to create, just affect