There are business and financial interests that control the parties. Nothing mysterious about it at all. They are the same kind of large interests that controlled British expansionism in places like southern Africa back when Cecil Rhodes was the big mover and shaker. It has to do with land, vital resources, commerce, industry, and military dominance. Very simple.
There is an aggressively expansionist policy. The parties serve it. The policy must be sold to the people by telling them whatever they want to hear at any given time, and the parties do that. When the public is thoroughly fed up with one party, the other one takes over, and the policy rolls on.
Now here's the key: The vast number of ordinary people IN the 2 parties are quite innocent, of course, of what's going on, and they do not realize they are being taken to the cleaners...every time.
If they did realize it then those 2 parties would collapse, and there might well be some form of revolution.
I don't see that happening, because the media will not blow the whistle, and people's impression of what's going on is secured by the media.
There are a handful of whistleblowers: Dennis Kucinich, Ron Paul, Pilger, Chomsky (to some extent)...that handful is not enough to stop the machine from carrying out its agenda, and its agenda is continual imperial expansion through lending money at exorbitant interest rates to poor countries (thus enslaving them), through moving our jobs to wherever the labor is dirt cheap, through laundering vast amounts of money, through moving drugs, and through provoking wars and by maintaining a high level of fear and paranoia in the public at all times....about terrorism, about crime, about drugs, etc.
The more fearful the public is, the more willing they are to give up their civil rights to "Big Brother" and to support foreign wars.
And that can only help the Empire to grow stronger.