In our primary schools, we learn our children mili, deci, centi, deca, hecto and kilo, and how to calculate between them.

Beyond that or below that is used either in science classes or specific usecases and not known by the whole population at large.

Since people use what they know, they’d never use mega as a common way of measuring. We mostly use km for distance, and only in specific cases we might use, say, hectometers or decameters.

5 megameter is not wrong, but I don’t call 34 cm 3,4 decimeters either.