Partly that may be because they can (in theory) work closer to the true customer, rather than having an additional contract/specification-based interface to getting things made. When you don't find out for six months that you completely misunderstood the client's needs, it's hard to build good stuff -- frustrating for the client, and for the developers.

If, however, you ARE your own client, such as writing research code or simulation/test code, you likely have a clearer idea of what needs to be done.