If we were to start that now, it would add only 1% generation capacity, and that's assuming that all projects finish within 13 years (unlikely) and that none of the construction projects are abandoned as unfinishable in any way that makes financial sense (which happened to 50% of the reactors started in the US in the 2000s).
There's an assumption that we can simply rebuild what we built in the past, but technology has changed quite a bit, construction costs have risen quite a bit, and the lessons we have learned from prior reactors means that we dont want to build the prior designs.
Nuclear is a technology without a solid track record, and which has failed in the US, and in France, and in Finland. In these latter examples, we can't blame regulations or public support. Personally, my hypothesis is that construction productivity has been so stagnant compared to manufacturing productivity growth, that nuclear no longer makes sense for advanced economies. Economies at earlier stages of development with lower labor productivity and therefore lower labor costs, may be able to build nuclear cost-effectively.