Comment by WorldMaker

Presumptive user feedback: "How come when I click this bookmark which has the icon nice and right there it sometimes takes minutes for the icon to show up on the tab?"

Principle of least surprises for the user is probably at play here. Bookmarks and tab icons seem like reasonably similar "chrome" to the user.

Separating the caches isn't necessarily easy either: it is just as likely to hand the trackers at that point a good signal for people who bookmarked a site based on whatever heuristic ends up being to refresh that cache if it is no longer "recently accessed tabs".