Also, digital, free-flow media lose basically all sense of space. PDFs are much better for finding a piece of content again later, because I can remember the location on the page and roughly how many pages into the document.
Though too be fair, reflowing HTML / displaying it without all the added-on cruft also often fails these days. Tools such as Reader Mode make heroic efforts, but also very frequently fail (or are apparently blocked or have sites blacklisted by tools).
Basically, for a bookmark to fully store a position, it would have to store all of the above (and probably more), and it would only be really usable on the same device as long as the underlying content does not change.
And I am almost always on the same device.
Any other scenario doesn't even ensure that the content will continue to exist, let alone have the same structure.
Zooming in/out should not trigger a reflow. Only things like changing the geometry of a page or font settings require a reflow.
It seems you're blaming a document format for a UX problem created by an implementation.