There are a number of reasons for the evolution of the risk-adverse culture. Some come from within the military, others originate outside it. Hanging it all on Congress is the easy answer, and ignores the problems that exist within the services. Doesn't mean that Congress doesn't have a share in the blame...but just that they aren't the only ones.

Constant rotation, a stunning cultural inability to look at its own history (but at least they've been consistent in that inability), accelerated deification of the military by outsiders with their own agendas, and a host of other things contribute to the military's inability to look at itself honestly and find answers. Congress...they're a mixed bag as well. If they pay attention at all, they remember the Cold War and its associated misconceptions and flawed assumptions about the US's historical military norms. They like spending programs, because those translate to jobs they can brag about and a certain level of myth creation.