The only reason why I find it very common is because they run out of money after playing civilian so much, so they just play cop when they have nothing to lose with that following role.
But the whole issue about it, is the current economy system that applies to both roles, hence why you have to rely so much on winning lotteries, or if you're desperate this much; then you would rob betting shops, and banks that pay 10k per each.
So, if this is about cops should be able to lose something, then they should earn a lot more, so people will play civilian more frequently after serving enough time with the law enforcement role.