Please do read again what you just said.
This thought process is the real problem.
The Nawaz Shareefs and the Zardaris were already billionaires then how come they continued with corruption? As per your logic they should have stopped...
There is a difference between the corruption of politicians and the corruption of civil servants. When you are greedy, you can be the richest man in the world but you will still like to have more money.
Nawaz Sharif's father was one of the richest industrialists in the country before Nawaz entered politics, and Zardari was the son of a landlord and a successful film distributor, although he also made money illegally when he joined politics.
My point is that Nawaz and Zardari didn't need to do a penny of corruption to live luxuriously, but they were slaves to their greed. These type of people exist in our bureaucracy as well, but there is also a signficant population who are not driven by greed but rather by necessity.
There are many, many civil servants who have no intention of doing corruption when they join the civil service, and they maintain their honesty for many years before coming to terms with the dark realities. Some times, you have to do favours for your higher ups and if you don't, you will be posted to some remote area as punishment.
Also, as you rise through the ranks, you achieve a certain status in society. Your social life changes, your wife starts to befriending rich people and you want your children to go to the best private schools. Eventually, because of the pressures of maintaining a certain lifestyle, you are left with no choice but make some money under the table.
Then there are people who come from poor backgrounds and by the time they are only 10 years from retirement, they realise that they barely have any savings for their retired life and to pass it on to their children when they die. What do you expect them to do in this situation?
It is very easy for people to sit in western countries, enjoy a high standard of living and make good money through legal channels and then pass judgements on how corruption is not justifiable in any circumstances and how it is the root of all evils etc.
The verdicts of these people on their high horses mean nothing because their morality has not been tested. When you are in a position of power and have the opportunity to make money without getting caught and you don't, then and only then do you have the right to speak against corruption.
Corruption due to poor greed can never be eliminated, but corruption that is driven by other factors can be minimised if their salaries are comparable to the private sector. As long as our civil servants are paid very little in spite of their efforts to be where they are today, the corruption in bureaucracy will always be there. Ultimately, it is a systematic failure and pointing fingers at individuals will not change anything.