My guess is there is some kind of momentum with these things. If everybody demands bribes, then by not demanding bribes yourself when you are in a position to do so, you are effectively pissing away your take but remember you still need to pay bribes to everyone else because they don't care you didn't take bribes.

On the flip side, if nobody else requires bribes but you do, you will surely stick out like a sore thumb. If I don't get paid bribes and I am an influential powerful person, why should I pay you any bribes? Especially for something that is legitimate and a part of your duties?