I guess it worked because everyone has an arm? dunno
Well, most people in biblical times were quite short, around 5’4”—5’5”. As long as they didn’t give the job of going around measuring things to Goliath or someone like that, it would probably would have been pretty consistent. Too, I’m guessing at the beginning of a building project, there would be an agreed upon standard cubit and they’d cut a few sticks at that length to use throughout that project. Or maybe there was an official standard that was already mandated locally. Something like that. Even as far back as the time of Hammurabi, they had building codes.