Business
When peaceful private buying, selling and making are outlawed, they do not disappear; they go underground and must pay off whoever can stop them. The corruption is the price ordinary exchange pays to exist where it is forbidden.
In a rigid planned economy, bribes and side deals are what actually move goods and materials between the links of the chain. Take them away and the machine does not become clean, it seizes up, like an engine run without oil.
Under central planning, official enterprises cannot meet their quotas through official channels alone. They too must pay gifts or bribes to get the materials they need to keep production moving.
Private production can run as a parasite inside official enterprises, using state workers on the side and materials taken from the state. The planned economy ends up housing the very private business it outlaws.
If corruption is what lets a forbidding, centrally controlled system function at all, no anti-corruption campaign can remove it. It will last as long as the system itself does.
Illegal resellers become a permanent fixture not because people are greedy but because they are often the only source of everyday goods. The forbidden trade persists because it does the job the official system fails to do.
Once tribute becomes normal, it reaches every stage of life: a bribe for decent care at childbirth, a gift so a teacher won't humiliate your child, theft from the bodies of the dead. Everyone, from top officials to people just getting by, is both giving and taking.
Where private enterprise is a crime, those who serve customers best can grow rich, but they hoard their wealth in gold, gems and art rather than investing it openly. However large their success, it can be wiped out by a prison sentence at any moment.
When corruption runs through the ruling apparatus, the security services know all about it yet the guilty are quietly removed rather than tried. Exposure becomes a tool for reshuffling power, not for enforcing the law.