That's intentional. It's so they can display correctly in portrait mode (by "correctly" I mean they fit on the screen). v10 default theme is not responsive (because v10 pre-dates many of the responsive features of HTML/CSS), so the columns cannot change their orientation automatically after a certain width point (e.g. indicating the user is in landscape mode).

I won't fix this as v10 needs to remain compatible with old technologies, and thus should not incorporate responsive features not previously incorporated. v11 is, however, responsive by default.