And during a dunkelflaute? (When the wind doesn't blow and the sun doesn't shine) Battery storage would be a huge undertaking for the lengths of time needed, plus the grid upgrades required would be astronomically expensive.
For example, the UK had a period of nearly 2 weeks of dunkelflaute in Nov 2024, where some areas had only 12 minutes of sunlight a day and winds dropped to 3-4% of the need. A similar event happened in 2021, so these aren't infrequent occurrences.
Although the lengths above are the extremities of the timescales of these events, we usually have between 2 and 10 dunkelflaute every year.
Nuclear is there to provide a flexible base load to prevent extended blackouts.