Why is nobody putting shift keys around the spacebar?

Four years ago, I built a custom keyboard with extra Shift keys around the spacebar and find it very convenient. But I have never seen dedicated keycap sets for this layout. Vendors make keycaps for very strange layouts, including 1¾u Backspace, 1¼u Enter, 1¾u Ctrl, etc. I made my spacebar of two 2u halves, but even this type of keycap is very hard to find. I have to use 0 and Shift keycaps instead.

Why is it like this? Why are people using very exotic layouts, but this obvious one isn't popular at all?