Gas turbines base load still take time to spin up and have lengthy shut down and start up procedures, even if they can be shut down. They are faster than a steam plant, but are not designed shut down and start up repeatedly over the course of a day.

The real question is why we are building any fossil fuel plants at all, and the answer is simple, they have immense lobbying power and vast full spectrum media campaigns that they use to prevent entirely viable existing alternatives from being built.