Because grid level power delivery is about FAR more than just raw wattage numbers. Momentum of spinning turbines is extremely important to the grid. The grid relies on generation equipment maintaing an AC frequency of 60 hz or 50hz or whatever a country decides on. Changing loads throughout the day literally add an amount of drag to the entire grid and it can drag the frequency down. The inverse can also happen. If you have fluctuating wind or cloud cover you can bring the whole grid down if you can't instantly spin up other methods to pick up the slack.

reliable consistent power delivery is absolutely critical when it comes to running the grid effectively and that is something that solar and wind are bad at

Ideally we will be able to use those technologies to fill grid level storage (batteries, pumped hydro) to supply 100% of our energy needs in the not too distant future but until then we desperately need large, consistent, clean power generation.