Each plant is unique, and not perfectly understood. Often the people that did understand it have forgotten, died, moved on, etc.

Understanding, improving, testing, and certification of a plan to protect against the huge risks involved is expensive and often in practice timelines and budgets often go significantly over.

There's numerous MUCH newer designs that: are much smaller, much easier to scale, absolutely identical, well understood, robust in the face of failure, and don't even need operators. Additionally since they are identical they get economies of scale and only need a finite number of experts on hand, not a group of them per site. You literally need a flat site, water, and electrical hookups. If you don't provide enough water for cooling they shut down. After their useful service life you put them back on a train car and ask for a replacement.

Some of these projects are ready to deliver, but the early customers have been cancelling. Bill Gates funded a project, and there's several around.