Why do people need dozens or hundreds of tabs to the point that they need groups for them? We've had bookmarks (and folders for them) for decades... It made way more sense when you could expect tabs to always be loaded, and not behave like another set of bookmarks.
Bookmarks have management overhead and are too easily forgotten about. For a lot of people tabs serve as a sort of to-do list or with groups, repositories for sites that will briefly be useful for a certain task in the near future. They naturally get closed as they’re done with, eschewing any kind of cleanup step as is necessary with bookmarks.
That sounds like how I use tabs but doesn't explain how people end up with enough tabs open to make them impossible to navigate in a horizontal layout and without grouping them. When I'm done with a tab, I close it. If I'm going back to it later, I leave it and... go back to it later. If I don't, I close it... or I bookmark it. I don't see how you go from using tabs as a short-term "read later" feature to having several dozens of them open at any given time. And while deleting bookmarks takes a second to do, so does going through your tabs to get rid of the ones that haven't been useful in 2 months. Personally, if I run out of horizontal space I consider it a good reminder to clean up, but then I see people with so many tabs that all they see is a ton of icons, and I wonder if they even know that bookmarks exist. That image came to mind as I read some of the other comments under this post, and I feel like we're just reinventing bookmarks for people who don't realize that bookmarks exist (bookmarks have been deemphasized for a while through trends like hiding the bookmarks toolbar by default).