On oval tracks why do the race cars always go counter clockwise and make only left turns?

Not all tracks are counter clockwise, but every NASCAR oval track has only left turns, because the drivers sit on that side of the car. The drivers generally try to keep to the inside of the track (because it is a shorter distance around the track) So by making all left turns, the drivers can better see what is going on around them. It is for safety reasons also; it keeps the driver away from the out side wall (guardrail in the old days).

In Australia, where they have a very active and popular oval track racing industry, the cars turn RIGHT, not left. This has to do with the fact that in that country the driver sits on the right side of the car, not the left as in the USA.

Despite some claims, it has nothing to do with centrifugal forces. Some link the direction the cars follow on track to the spin of water going down a drain- supposedly, the spin of water in a drain is dictated by the earth's rotation and varies depending upon the hemisphere, north or south. The entire drain theory, however, is a myth; the earth's rotation and being in a certain hemisphere has no bearing on either water in a drain nor the forces exerted on cars on a track.