You're always centering children with respect to something, and you specify the fact that you want children centered on the thing you're centering them in, rather than the elements themselves. This makes more sense, in my opinion, and as said in the sibling comment, it's two simple rules.

I do a tonne of flexbox and grid in my day-to-day, and these complaints are quite simply out of date. Almost any layout in CSS these days can be done easily and intuitively.