They can.

In many places, a lot of the time, they can. But not everywhere, all of the time.

The problem is that if even fellow environmentalists like yourself keep thinking of nuclear like a boogeyman, or just not knowing how a power grid works, then we stand no hope of decarbonising power generation. Did you know fossil fuel use is growing worldwide in electricity generation while nuclear is stagnating? The way to decrease that that brown area is by increasing all the other colours in similar proportion. There are circumstances where a fully renewable grid is possible, but those conditions aren't the same everywhere, and those niches will continue to be filled by fossil fuels until we stop being afraid of the much better alternative. The fact is countries with higher percentages of nuclear in their power mix have much lower emissions per GDP per capita than their neighbours. So I may not be able to convince you, but I'm going to keep trying to educate anyway.

Tell me, then, how can you have a stable grid with renewables alone in places where (or when) pumped hydro isn't feasible or can't provide enough power by itself? Or in countries prone to lengthy droughts, like my own? I'm not asking this to argue, but because our disagreement may come from a misunderstanding of the base working principles of the power grid.