The point remains that centering vertically with absolute positioning does not work with an element of variable height. Meanwhile, horizontally centering something of variable width is dead simple. Why the difference? It's a shortcoming of CSS.
There is a fundamental reason for the difference: in standard CSS block/inline layout, widths depend on parent widths, but heights depend on child heights. Far from being stupid, this difference is the entire reason CSS is parallelizable at all.
Flexbox makes this simpler by switching to an entirely different model, which is the right approach for UIs (as opposed to documents). But I can't really fault CSS 2.1 for not introducing flexbox right away, because CSS 2.1 is complex enough as it is.
