Just some theories:
Theory one: small airplane pilots can't afford/understand pocket calculators. They can afford tape measures, and they can reach the datum plane (usually the firewall).
Big airplane pilots can afford/understand calculators, but can't find the datum plane (often an imaginary surface some odd distance in front of the airplane).
Theory two: little airplane designers can't afford/understand pocket calculators, but big airplane designers can.
It's probably theory two, since most pilots don't really know where the c.g. is: that is, they couldn't point to it on the airplane. It's just a number, that is either good or bad.