Well, you have to handle excess power produced, you can't just dump it on the ground.
If the grid produces too much power in excess of what's being consumed, parts of it need to shutdown to prevent damage.
That's why the price can go negative. They'll actively pay you to use the power so they don't have to hit emergency shutdowns.
As we build more solar plants, the problem gets exacerbated since all the solar plants produce power at the same time until it's in excess of what anyone needs. Unlimited free power isn't very helpful if when it's producing it's producing so much that it has to be cut from the grid, and when demand rises it's not producing and they have to spin up gas turbines.
That's before the money part of it, where people don't want to spend a million dollars to make a plant that they need to pay people to use power from.
https://www.technologyreview.com/2021/07/14/1028461/solar-value-deflation-california-climate-change/
They go on to talk about how getting consumption to be shifted to those high production times can help, as can building power storage systems or just ways to better share power with places further away.