This is because I basically use tabs as bookmarks relevant to a project or subject area. Bookmarks are also tree-structured, but are much more high-ceremony to create.
To my mind, tabs and bookmarks should meld into one. If you don't close a tab actively, it stays deactivated, its tree likely gets collapsed until needed, so it's not an eyesore. When you need it again, it's there, in the proper context.
If you close a tab, it goes to history. But a tree view of history is possible, too (there are extensions for that), so that you can track, from which page did you open this link, what links did you open on this page, etc.