It is like the apochryphal caged monkeys experiment. People learn that corruption is then only way to get ahead, and who wants to be a martyr? One person can't change the system, some sort of massive coordinated action would be needed-- but how? Hence, rebel groups and freedom fighters and whatnot, but those have their own problems.
There are some people in this thread who suggest that the problem is that people accept the status quo. That if nobody would pay anymore, the system would fix itself. There's a huge cultural gap that makes this hard to understand, but people in these countries don't want to end the exploitation, they want to become the exploiters themselves! The mechanism that allow the system to continue are reinforced by the new players. There's a feedback loop in place that makes sure the system accepts only a certain kind of new players.