There's more cognitive effort needed when creating a bookmark (not to mention several clicks and key presses) - I need to classify, organize, assign a folder, think about relevant tags.

More importantly: while I expect to need the tab again in the near future, it is likely to be completely useless in a few days. Creating a bookmark for that is going to be wasteful clutter that I need to spend more mental energy cleaning up.
The parent comment expressed incredulity at being able to manage that many tabs. I'm not sure how converting them to bookmarks instead helps. It just seems like I'll need more clicks to get to my site.

Also, I care about the state saved in my open tabs and don't want to reload the page every time I visit it. Many websites are built in such a way that loading the URL again doesn't even restore the same state, and sometimes it doesn't work at all.

Bookmarks are useful, and I do use them, but they are not a workable replacement for tabs, at least to me.