People's informational bits are not unified as html or pdf. They could be anywhere: html on web (more than one browser), html on mobile (which browser is it?), pdf here, pdf there; tons of apps icons, whatsapps, facebook, bookmarks galore, blah, blah. We are all over the place. Annotation on the other hand is a tool for organized individuals. Life is not necessarily organized for most people, hence, this never got picked up.

Annotation should have been part of the browser from day one. It should have evolved with it when things moved on to the mobile world. It never happened.