Many IC manufacturers treat their "API" (hardware registers) as proprietary IP, require an NDA from the driver developers before releasing the development guide, and require an agreement not to publish the driver.

I'm mostly referring to the smaller support chips here. Things like I2C->I2S bridges and ADCs and other crummy little parts that every board needs, and that need small drivers. The big ones, though, would be WiFi, Bluetooth, and cell radio chips. They can be very strict.

Even if Ubuntu had the time and money, they might not have the rights.