[pinebox]

Because router vendors need some kind of "secret sauce" in order to keep selling routers, otherwise the whole thing becomes a commodity you can buy from anywhere. This is also why OpenWRT is unable to utilize all the features on a lot of routers: The closed source firmware and software pieces needed to do so have yet to be deciphered.

[citrin_ru]

Vendors can ship OpenWRT + closed source drivers. Anyway they use Linux kernel (yet to see a closed source BSD router; PfSense is occasionally used but its open source) relying on GPL exceptions allowing to use closed source drivers (and firmware blobs).