Why not just build the wind/solar/storage to cover it all.

If that’s too expensive why not just build enough nuclear to cover it all.

Because they do different things.

Suppose you need 10GW of power for an absolute baseline. Enough to heat homes to a temperature that people don't freeze to death on a cold day, to keep power to hospitals and other critical services, etc. Then you need another 10GW on top of that to run aluminum smelters and heat homes to 80°F instead of 60°F and things like that.

If you have 20GW (average) of wind but you get an extended period of low generation and the batteries run down, people die. If you have 10GW (average) of wind and 10GW of nuclear and you get an extended period of low wind generation, the price of electricity goes up that week and people turn off their aluminum smelters and things but nobody dies. If you have 20GW of nuclear you can run the aluminum smelter 52 weeks a year instead of 51 but then people are paying more for electricity than they would with renewables in the mix, which isn't worth it.

So which one should we do?