Because some people are always in favour of cutting corners, bending the rules, taking the easy way out, accepting money for a job they are already paid for, taking other people's money and so on.
Money changes people even though not many are willing to recognise it or admit it. This is really, has been proven many times.
Once a politician said politicians should be chosen based on wealth as well, meaning a politician should have some wealth when entering office so he/she won't be tempted to accept bribe money or take cuts for personal use. This is a good idea, however there are people who are never satisfied with anything, no matter how much they have, they always want more.
The other side of the story is justice system. When there are well defined laws, without loopholes and those laws are applied to the letter, corruption is minimal. The problem starts when these people who should make sure the law is applied are the most corrupted.
Corruption thrives because corrupt people are not being brought to justice : they always get away. When an evil is allowed to go on unpunished, such evil is likely going to continue for a very long time. If very stiff penalties are attached to corrupt practices and anyone found wanting is given the necessary punishment regardless of status I'd position, corruption would reduce drastically. But if nothing is done to corrupt people and they are allowed to flaunt their stolen wealth to the people, everyone would see corruption as acceptable and want to take part in it.
A strengthened judicial system and a stiffer punishment for corrupt people would reduce corruption to a minimum