The statistics say otherwise. GA pilots have that option, but on a smaller scale. Those who invest in themselves can frequently approach the airline level of safety because they work at it. The VFR private pilot operating in light winds in the local area on nice days can operate quite safely with relatively little training as long as his basics are good and he is proficient. However, when you start to venture forth from the home drome and into various weather systems with new geography, the need for regular competent training ratchets up quickly if a comparable level of safety is desired.
Regulation has a significant effect on the airline pilot's decision-making process.
Many of the decisions that a GA pilot is faced with are already made for the FAR Part 121 pilot. For example, when the weather is below landing minimums as the flight arrives at the final approach fix, it's either on to the alternate or hold for improvement. In GA, the pilot gets to decide whether to shoot the approach. The risk is slightly higher, along with the temptation to cheat. There are many more constraints placed on the airlines, and the paying public is entitled to a higher standard of care.