Because not only do not ALL users use lvm2, but MOST users who use lvm2 do not use the /dev/vg/lv scheme and instead use /dev/mapper/vg-lv or UUID= (UUID= has always been the ubuntu-preferred method)
Because not only do not ALL users use lvm2, but MOST users who use lvm2 do not use the /dev/vg/lv scheme and instead use /dev/mapper/vg-lv or UUID= (UUID= has always been the ubuntu-preferred method)