Because humans always feel lacking and dissatisfied with what they have achieved. Whether it is enough or not depends on how one looks at it. People who always feel like money is never enough are like thirsty people who drink salt water. The more you drink, the thirstier you become. Even if you drink how many liters of water you will still feel thirsty. People who always feel short of money, no matter how much money they have, will never feel like they have enough. For example, when you are poor you will do anything to be rich, or at least look rich. When you are rich you will look for ways to be richer than the rich. Finally, the only thing that can stop the feeling of always lacking is death.
Yep, that's part of human nature, it's just that some people manage to control it and some can't help it. This feeling of having more becomes stronger when you have some, for example, if you don't have money but then you manage to get some money from somewhere, you start to feel that you should have more after that. One might feel that they will be contented once they get more, but in reality, that doesn't happen and the thirst keep getting bigger and bigger as you said.
People that are poor think that those who have a lot of money might not need more or desire for more, but that isn't true. The rich keep trying to become richer over time, so this struggle never ends.