Bookmarks are perceptually longer-term than open tabs, so there may be more reluctance to save to a bookmark. (E.g., if planning a trip to Italy, do you want to bookmark some blogger's food recs for Rome, forever?)
But worse is, it relies on recalling the text in the bookmark's title to resurface it. You might not remember the page title, but you can always scan through open tabs.
This is an interesting idea for a feature, that I think I would like too. I like to save things to maybe look at later and a TTL would manage automatically dropping them from bookmarks in case I never actually want to look at it later.
I add a folder to my bookmark bar. All project related tabs get bookmarked there. When I'm done, I either delete the whole folder or file it somewhere.
Work make me use Chrome, and I have recently converted hard to tab groups. I've found two main uses: one for a collection of reference tabs that I mostly want open or closed together (specific API references that are normally spread out over a few pages); the other is to organise groups of tabs for different projects I'm working on.
Both of these make context switching easier as I can quickly hide all of the tabs I'm not currently using, knowing they'll be just as easy to reopen later. In Chrome, tab groups can be saved too, so they give you a bit of the persistence of bookmarks.
I'm still a Firefox user where I have a choice, and I'm really excited to hear they're working on first-class tab groups
Think of it like memory hierarchies. Bookmarks are long term storage, tabs are registers. Tab groups fall somewhere in the middle, easy to reengage with and easy to put out of focus.
Bookmarks suck. They are slow and cumbersome to manage, especially when it's many related urls. And for working with them, I need to open them as a tab anyway, so why not stay there from the beginning?
But worse is, it relies on recalling the text in the bookmark's title to resurface it. You might not remember the page title, but you can always scan through open tabs.