When you have a connected grid, it needs to ensure the entire grid operates on the same frequency. Production and consumption affect the frequency, so they have to be perfectly balanced. This is a hard problem when it needs to be done at scale of entire countries or continents.

Traditionally this has been managed basically in an analog way by having huge fossil- or nuclear-powered steam turbines that spin at the desired frequency. Apparently a spinning mass is so good at stabilizing grid frequency, that massive flywheels are being added to the grid to replace the role of the fossil turbines.

Our old power grids make solar and wind production merely follow the existing grid frequency, rather than dictate one, so at least in our grids pure solar couldn't work. I don't know if there's a solid technical reason for it, or that's just how it's been set up.

That's a global thing. Batteries are simply too expensive and not available in sufficient quantities to provide reliable base load power for heavy industry. Perhaps that might change in a few decades but that's the reality today.