Especially considering that most of the time, vendors don't make any interesting changes to GPLed code in the first place! They ship unmodified busybox, slightly patched Linux with a few strange drivers, and so on. Shipping source code for the GPLed components would cause them more or less zero disadvantage, and the first handful of vendors to do so would also get some kudos from the FOSS world. Other than incompetence, I don't see any obvious reason to violate the GPL in most cases.