Because it can differ from person to person. Like a 12 year old will havea smaller foot and smalller cubit than a 40 year old man
HOPE IT HELPS!!!!!!!
Keshang Yolmo answered this
we cannot because the lenght of a cubit is not accurate.........
Shreyas answered this
we cannot use it because they vary from person to person
Shruti Swaminathan answered this
Foot or cubit can be used as a standard unit of length as they are inaccurate and vary from person to person. It's not helpful if we want exact measurement. That's why the SI unit of measurement was developed by scientists from all over the world.
Afrahkhan answered this
this is because every persons foot and hands differ so to be appropriate it is important to follow the standard units.