Yes, that’s possible, but adding a bookmark, then replying, then removing the bookmark again is a lot less convenient than having direct access to the tracked threads. And I suspect keeping the bookmark in addition to tracking would not only still be double effort, but also clutter the bookmark list quickly.
In addition, but that might be a “me problem”, my mental model of bookmarks is that they are for permanent/long-term access to relevant information (e.g. documentation threads here on Meta), while the tracking state is more ephemeral.