For the same reason that using things like a MAC address is a bad idea. You really want to be
sure the data from /dev/urandom is not only random but secret too. There are services such as
http://www.random.org/ that provide really random numbers, but they are aimed at scientific
and statistical applications rather than cryptographic uses.
Also the original problem relates to randomness available to an installer, where I'm sure the
network is unconfigured.