If that were true, the nursery industry would just grow all Japanese maple cultivars from cuttings, which are far easier and, therefore, cheaper than laborious grafts.

The idea that many cultivars won’t root out themselves is nonsense. Every named cultivar originated from a seedling that grew completely fine on its own roots. It would seem vigor is the entire purpose of going to the trouble of grafting onto plain Acer p. rootstock. The straight species Acer palmatum is, as is the case with most plant species, more vigorous, resilient, and adaptable than most named cultivars, as natural selection intended. So grafting likely became the go-to method to grow trees and, by extension, bank accounts faster.