In order to simplify, we are going to use 'cleaning your room' as a good example of that complete purity is very difficult to achieve on Earth. It takes a lot of energy to do it (cleaning your room) every single day, always maintaining it absolutely clean even in the most inaccessible places; of course, people doesn't have enough time to do it daily. Complete removal of corruption of any kind is on the same level of difficulty or even more!