That idea sounds great, but it doesn't work with the current paradigm of "It's our phone, you're just paying us for the privilege of using it."
That's the most toxic aspect of the current mobile device environment. Doesn't even matter if you buy the phone outright, many carriers/handset makers choose to use permanently locked bootloaders or otherwise obfuscate the process of installing a custom ROM.
Add to that every device has wildly different drivers/hardware supported only by the handset makers themselves and you have a very unfriendly environment indeed for custom ROMs. Half of your phones whizz-bang fancy features might not work in CM because they're mostly implemented by the stock ROM. The camera in my LG G3 is garbage in CM, all the fancy features are in stock and the stock ROM is fucking awful.