We plan to fix this in Chrome, see
Spec discussion: https://github.com/whatwg/html/issues/2851.
Expected: Scrollable divs have tabIndex=0 and can be focused using sequential navigation.
Actual: Scrollable divs have tabIndex=-1 and cannot be focused using sequential navigation.
Making scrollable element focusable might be good for UX but it has a major implementation challenge, which is that this would require computing the up-to-date style for an element to decide whether a given element is focusable or not.
We're actively tying to avoid updating the style to decide focusability of an element, and this goes directly against that goal.