Dave, we know the Sun  goes down every day.  We also know, usually multiple days in advance, when clouds will block sunshine.  We know hours and days ahead when the wind will blow.  We know how much wind and sunshine to expect per week and per month during the coming years.

We do not know when one or more reactors might suddenly disappear from the grid.  And they do.

If a reactor goes offline it will generally be offline for days.  It can take days to restart a reactor.  If the needed repairs are extensive a grid can end up with one or more reactors offline for weeks, months or more than a year.

It is a necessary assumption that the wind will not blow 24/365, the Sun will not shine 24/365, and a nuclear reactor will not be operational 24/365.  Grid design involves building in the needed storage/backup to deal with those events.

The question is cost, is it not?  I gave you some numbers to consider.