Every reactor and every plant is bespoke, even if they are based on a common "design" each instance is different enough that every project has to be managed from the ground up as a new thing, you get certified only on a single plant, operators can't move from plant to plant without recertification, etc
Part of that is because they are so big and massive, and take a long time to build. If we'd build smaller, modular reactors that are literally exactly the same every single time you would begin to get economies of scale, you'd be able to get by without having to build a complete replica for training every time, and by being smaller you'd get to value delivery much quicker reducing the finance costs, which would then let you plow the profits from Reactor A into Reactor B's construction
You can also build standardized, modular LARGE nuclear power reactors. The French and the Japanese did it and managed to builds lots of large reactors with relatively short build times