Because we simply are unable to do so fast enough. China recently started a massive, largest in the world project to add 150 plants over the next 15 years. That's 10 reactors annually, or an estimated 10GW using the current 49.6GW from 50 units they have already.
Adjusted for capacity factor this is maybe going to keep up with solar deployment, but definitely not with wind capacity growth in the same country. And China has been the world leader in nuclear deployment for a while now.
Renewables win on a "worse is better" basis - yes, they're intermittent, but they're cheap and deploy in a matter of months without too many specialists involved.
You didn't explain why we shouldn't do both. Just because it takes a long time doesn't matter. If we want to ensure we have consistent energy production then nuclear should be built. Even if we can build a massive amount of storage it may not be enough. Global warming is supposed to cause extreme weather. Why not create nuclear which can work regardless of weather?
Adjusted for capacity factor this is maybe going to keep up with solar deployment, but definitely not with wind capacity growth in the same country. And China has been the world leader in nuclear deployment for a while now.
Renewables win on a "worse is better" basis - yes, they're intermittent, but they're cheap and deploy in a matter of months without too many specialists involved.