The situation that the use of drugs in sport most closely resembles is that of criminal theft. Someone sees the huge amounts of money that other people are making and tries to get some for themselves. So we need to start by trying to understand the way the mind of a thief works.
Human beings love certainty.
In the same sense even though the criminal is considered a social outcast in lot's of ways they still have the same basic drives as the rest of us. Sure there's the adrenalin rust for a few minutes but after that the coast is clear. Not surprisingly it isn't that different to the petty criminal above. Generally they will know someone who has used drugs and has gotten away with it and if they can get past the initial testing and win the gold medal then they're in the clear. Besides the people they have heard about that have been caught haven't even been severally punished anyway.