Take a look at Wind River or Montavista's web pages, they're so full of glossy marketing stuff
and buzzwords. Instead of saying the truth, "for some cash, we can save you a few weeks of
work in bringing up your Linux board", they're trying to build up some kind of hype.
I think a lot of people decide to roll their own system (and with the help of projects like
Buildroot it can be quite painless, if you're lucky). In part this is because they're trying
to save money, but it's also an uncertainty on how much value these commercial vendors really
add. You can compare the situation to the desktop Linux world, where many people (me at least)
prefer upstream fixes over distribution-specific add-ons.
Going from a modern PC Linux distribution into the embedded Linux world can sometimes feel a
bit like going back in time 10 years. It can sometimes be a bit rough around the edges, but
for an engineer that's also part of the fun. The joy of bringing up a board for the first time
and seeing the boot messages scrolling by over the serial port, priceless!