Tabs store context (history, cookies, container, etc, with tree-style tabs also parent & child tab relations) that bookmarks don't. I'm not sure about Safari (since I don't use a Mac) but on Firefox you can set up tab discarding (via extension) that unloads unused tabs. So there's a lot less incentive to use bookmarks when you can just leave a tab open forever. Really the only reason to use bookmarks instead of tabs is for synchronization across devices.