Canon simply follows very different goals with their two mirrorless mounts. Making them compatible wouldn't just be difficult, it would make little sense, because they are so different. Who wants to use the huge RF lenses which clearly aim at pushing the limits of optical quality on a camera system that puts size so far up on the list of priorities that all lenses for that system have the same small diameter? Probably some people, but they simply aren't who Canon targets with the M system. The adapter is there so to easy people into the system and not be forced to make a M version of the lenses that wouldn't benefit from the mount, I guess. I wonder how many people use that to pair their M with large primes and fast zoom lenses regularly.
Even if both mounts would share 20mm flange distance, they would be incompatible. Canon EF and RF share a 54mm diameter, the M mount is smaller at just 47. In other words, adapting M lenses to RF would work with an adapter that sits inside the mount, but to go the other way you would have to add some length. The way it is now, an adapter isn't completely impossible, just very unlikely.
The M mount is designed so that Canon can make tiny lenses for it, without making too strong compromises to quality or price. Clearly RF is just the EF mount with added flexibility. It's a thicker mount, so any lens made for EF/RF will at least be larger at the mount than any lens made for the M system. Being small it what sells the M system so Canon clearly went with the right mount there.
I think it is inevitable that Canon will release an RF APS-C camera if they want to transition models like the 80D or 7D II to mirrorless. Unless they can push down the cost of high resolution FF to match those APS-C DSLR, maybe.