The usability of bookmarks is far worse than that of tabs. With tabs, I can just middle click in the relevant window and the tab will be in the right place, waiting for me. With bookmarks I have to either spend a lot of effort putting the bookmark into the right folder, or just have one big uncategorized list of bookmarks. And I'd have to remove the bookmarks once I'm done with them.

And as someone who opens up hundreds of tabs and returns to many of them only several months later, bookmarks serve a different purpose in my workflow than tabs. Tabs are single-use; I close them after I consume the content in the tab and never open them again. Bookmarks are reusable, pages which I may want to return to multiple times.