It's deliberately absent because the outrageous amount of repetitive and thankless support work it generates is/was (in our opinion) a bigger problem than the increasingly negligible speed difference between most eMMC storage and a decent SD card.
Running LE from eMMC storage isn't a bad idea, but the current implementation and user experience with that script *is* a bad idea. At some point it will be tackled in a way that makes sense over Allwinner, Amlogic, Rockchip, etc. .. but some features need to land in uboot before we make any serious efforts on that, it's a little too early.