Why would auto tires have traction on dry roads but not on wet roads?

I've notice lately that when the road is dry, I have no trouble with traction, but even on slightly damp roads, it's like driving on black ice. Turning corners feels like my car is skidding and on accelerating from a stop, the tires spin briefly before traction is established. I examined the tires and the tread looks fine and the pressure is within suggested parameters. They only have about 20,000 miles on them. Any opinions?