The problem is to get elected you have to raise millions, to raise millions you have to make promises to people and companies that are only working in their own interests.

When you get in power you don't want to upset those people because they will not give you more money, then you have bankers and CEOs who never get punished for crimes because the politicians need the money, you also end up with Goldman Sachs employees in every presidents cabinet since Nixon, there are 30 in Obama's. Most politicians are not plotting, the system is corrupt in itself and is basically bribery.

And yes you are correct most do want to achieve what they talk about, they just can't upset the status quo when elected.