Perhaps NASA's risk aversion comes from many things. For one thing, it seems that we've become more risk-adverse as a society. Second, there are only 3 Shuttles left and there won't be any more built. Third, bad PR can lead to a public backlash, which could result in budget cuts.

As for your statement, Perhaps it is money. If NASA had a blank check, it probably would be willing to take more risk. It works for the air force and their R&D for new planes.

Back in the 1960s when NASA's budget was more than twice todays (when adjusted for inflation), they had a motto, "Waste anything but time." They did take a lot of risks and developed an attitude of "Go Fever." The result was the Apollo 1 fire and several near accidents in earlier launches.

As for the Air Force, their budget is hardly a blank check and they've become pretty risk adverse, too. For decades, it was accepted that there would be losses during flight test. It was just the cost of pushing the edge. If you look back through aviation history, you'll find that many planes that went on to have outstanding careers had lost at least one of the prototypes (P-38, P-47, P-51, B-17, B-29, F-100, Navy F-14, F-16 to name just a few off the top of my head). As a result, virtually every street and building on Edwards AFB is named after a dead test pilot (as is the base itself). As the planes became more expensive and the Congressional attention greater, they stepped up safety programs to the point where flight test losses are quite rare and fatalities even more so. Offhand, I can recall that a B-1A prototype was lost in the 1980s and the YF-22A prototype was lost in 1992. I don't know of any other examples (at least involving manned planes intended for production) recently. The Navy/Marines lost some V-22 Ospreys and there were some fatalities, but too many other flight test losses come to mind.