No because fast changes damage the rods, the fuel, and the pressure tank, to many fast cycles would cause the reactor to get damaged and a damaged reactor is one that is damaged forever, nobody can repair the stuff and pressure damage is hard to repair.

I didn’t say you can’t change the output, but doing that is not economically and ecologicaly a good thing. Also the change speed isn’t fast enough for a electricity grid, a grid needs to hold a constant frequency all over the grid and balance throughout it, peak and low demands are very frequent and thats not possible to cover with nuclear at all, because the output changes are way to slow.