Perhaps they could...but they haven’t. But let me get this straight, you are basing your lack of understanding of the M system’s appeal on the current lack of some hypothetical future lenses Canon may or may not produce? Truly, you have a dizzying intellect.
Even if Canon does come out with small lenses for the RF mount (slow, variable aperture zooms that have just decent optical quality), the system will still be substantially larger than the M line
More importantly, smaller RF lenses won’t detract from the other major reason for the appeal of the M system:
Next you’ll probably claim that Canon could make RF lenses and a FF MILC cheaper. Before doing so, you should consider the fact that the for the same price of the slow variable aperture, medium image quality EF 24-105/3.5-5.6 STM, I can buy an M6 + M15-45 kit.
But it’s ok if you don’t understand the appeal, enough people do to make it one of the most popular MILC lines in the world, and the most popular mirrorless line in Japan.