They are more expensive only when compared to a marginal kWh put at time of solar/wind’s convenience into the grid that is stabilised by other means (mostly fossils). Look up how much electricity went up in Texas last time the generation dropped by iirc just ~20%; solar goes to zero every night, and wind regularly dies down across Western Europe for days or even weeks. In those circumstances and without fossils to fall back to, nuclear is infinitely cheaper than solar/wind.

We can’t plan our grids or judge the costs according to not just best, but even average scenarios. We have to plan once-in-decades anomalies, the long tail. Non-dispatchable energy sources get exponentially more expensive once you start doing that.