It's not a steady supply over power, it fluctuates with your supply and demand for it, so it takes work for another grid to take that power from you. If you send them power they have to scale their plants down to prevent the problem you're having. Then when your supply goes down and you no longer have an excess of power to send they have to scale their systems back up to make up for the power you're no longer sending. They can't store it or get rid of it easily. It's one of the reasons people don't like wind and solar power, it works great when the wind is blowing and the sun is out, but it fluctuates too much and causes heavy swings in power generation that have to be dealt with somehow.
