Camera manufacturers live and die on their reputation for making tools that deliver for the professional users of those tools. On a modern camera, the firmware and software needs to 100% Just Work and completely get out of the photographer's way, and a photographer needs to be able to grab a (camera) body out of the locker and know exactly what it's going to do for given settings.
The more cameras out there running customized firmware, the more likely someone misses a shot because "shutter priority is different on this specific 5d4" or similar.
I'm sure Canon is quietly pleased that Magic Lantern has kept up the resale value of their older bodies. I'm happy that Magic Lantern exists-- I no longer need an external intervalometer! It does make sense, though, that camera manufacturers don't deliberately ship cameras as openly-programmable computational photography tools.