Because a free market is a bad thing and no one alive can actually remember the last time we had a free market. There is a REASON we don't have a lasse fair market. Left to it's own devices a truly free market will do horrible things to people AND have huge swings between booms and busts. We don't want our GDP to look like a sin() wave. We WANT regulatory bodies to slow growth when it is happening too fast and spur growth during recessions. This is all econ 101.

Also, we have learned a lot about economics since the days of truly free markets. Hell, we have learned a lot about economics since 1930.