Pure speculation on my part, but some of the reasons that come to mind.

  White collar crime is punished less severely then homicide.
  The parties involved don't want to get into an arms race that isn't already started.
  There is plenty of ways to make a problem go away, including the FEC and IRS.
  No single party has complete control over the government and legal system.

While I generally disagree with lobbyist, I think (at least in the United States) the extreme business interest pressed on the government tends to keep some things in check by each business demanding counter-acting changes.

The ones winning are the politicians, the ones loosing are the under-represented (the individual)

Edit: For certain values of winning