I don't know why that would be the case. Why would we build nuclear plants that produce too much power when we could build the amount that just produces a certain portion of our power and that can run wide open so that no excess need be sold off. France just put too much of its power generation in a non-flexible energy source. If it had only build enough for 50% of its power, it probably would not have a problem.
Power consumption is always changing moment by moment. If you ever get the chance, try to get a tour of your local electric utility’s control center. For Progress Energy in North Carolina, it’s the Skaale Center on Hillsborough St. in Raleigh.