As I've been thinking about this and reading all your responses, something occurred to me.
Quality matters.
Styling matters.
If both those things are true, then those with a smaller wardrobe are at a distinct advantage for this type of dressing. For those of us who don't have a stylist on our staff, having fewer, higher-quality items not only means owning pieces that fit and lay better, but having more practice styling each item we own. When we're adding an endless stream of new pieces, we're constantly reinventing the wheel, aren't we? Rather than learning the ins and outs of a small number of wardrobe items, we're fiddling with endless combinations and, possibly, never perfecting any of them.
Maybe those who "just throw something on" and look fabulous are able to do so because they've worn that exact combination thirty times before and know exactly the right way to tuck the top and cuff the hems and scrunch the sleeves for best effect. No experimenting necessary because it's already been done.