I think it's less about people working in their hands than what we grew up with and how we learned to "measure the world". Everybody knows how long/wide/heavy etc things are in his/her own system and finds convenient ways to measure anything -when I spread my hand I know it's more or less 20cm from the tip of my thumb to the tip of my middle finger and I can measure anything by hand; also, we know X is more or less 6 feet (1.80m) tall because he's about as tall as Y who we know he is and not because we actually measured him. We also have mental images of what our measurement units are. To illustrate it's not about metric but about what we compare things to, how we comprehend measurements in the first place and how we experience them in real life, francs went in metric and euros do too and I can't help converting euros in francs if I want to know how much things are actually worth. For big amounts, I need to add two zero's to get the amount in cents so I know a house is worth so many millions cts and then I feel how much it is. Besides, we all share years, days, hours etc (not easy stuff to calculate quickly either!). Don't we all feel younger than what we actually are? Wouldn't it be because we buit our own inner scale of what a life time is and what people look like at 20, 30, 40 etc we were young and so older people looked much older, and now we don't feel we are like those "old" people we kept in mind? Metric is muuuuuuuuuuch easier to deal with because it's in base 10 and it's all about adding or removing 0's at the end of a figure or adding, removing or moving a point or a comma depending on your standard.