Because citizens allow the political authorities to operate in that manner. The majority of people are apathetic and uninformed, allowing corruption to proceed without hinderance and often not caring; so long as it doesn't have an immediate effect on their personal life, wealth, etc. People are usually either consciously or sub-consciously aware of corruption and dirty politics. The fact is, if their life continues smoothly and as usual, they're happy to ignore that corruption.
Where corruption can exist, it will exist, unless actively rooted out. People turn a blind eye, however the more uncomfortable corruption makes people's lives, the more willing they are to do something about it. Inversely, when a population becomes too complacent; corrupt groups in their governments can more easily play a 'dirty game.' I believe decades of being apathetic and complacent towards politics is finally coming back to bite us.