Because the people that own the gas stations want to make more money then they need. From
Greed on the part of oil companies and their corporate bosses. Station owners are at the mercy of the supplier - if the gasoline supplier raises its rates, the station owner must pass those along to the consumer or face going out of business. The station owners profits are not as high as one would think.
The other reason is the simple 'supply vs demand' thing. As long as we continue to drive more, and drive gas guzzling monsters, there will always be a high demand for gasoline. The greater the demand, the higher the price - pure and simple. Only 20% of our oil resources come from foreign countries - the rest is manufactured in the US, so we can't really say that foreign countries are to blame - we are doing this to ourselves.