Why don't they put solar panels on wind turbines?

You mean place solar panels on the tower, nacelle, or blades? On blades: doing so adds extra weight on blades, and hence reduces the power output of the wind turbine. read more

On blades: doing so adds extra weight on blades, and hence reduces the power output of the wind turbine. The solar panels would be constantly rotating with the blades, so they will face sun in less than half of a rotation cycle (if they are not shadowed by other blades). read more

It is actually happening as we speak. The issues involved are cost of the wind turbines and solar panels- falling steadily- and the cost of oil and gas- rising steadily. The cost of renewables is probably around twice non-renewables right now, in most places. So it might cost double the price of oil or gas for the big turbine farm or solar panels. read more