Because there's a minimum demand you must be able to supply.

Here in Norway we get just about all our power from hydro, and we have a lot of pumped storage lakes which we use as "water batteries". However, eventually hydro relies on water falling from the sky.

Not long ago there had been some really dry years, and our storage was running at record lows. Had the subsequent year been dry we'd be in a real pickle.

Another aspect here is that production is one thing, but grid-scale renewable production rarely happens right next to the primary consumers, and has to be transported. And the grid might not be able to.

Again here in Norway, we had a situation not long ago where the price difference between the north of Norway and the south of Norway was 100x because the south struggled to produce while the north was overflowing, but there was insufficient capacity on the grid to send all the energy being produced up north down south.