Because of the uncertainty of how much power your solar panels will have available, the provider must maintain sufficient margin available to handle the instantaneous load of the entire system.
It was always true that the provider had to maintain sufficient margin between electrical supply and demand in order to prevent blackouts. And yes, now they have to look at weather reports to determine not just demand but also supply--although the two cancel each other out to a degree because people use less A/C on cloudy days.
But providers now have one additional tool to manage demand: smart meters. Conceptually, you can program your smart meter to raise your thermostat temperature in the summer during periods of high electrical demand, or lower your hot water temperature, or turn off lights, or tell your washer to refuse to start a load of laundry until the demand event has passed. As a result, providers can now reduce demand within seconds where it otherwise takes 10 minutes to bring a peaker plant online, and this reduces the margin providers need to maintain.