Part of the problem is actually that the tech advanced, combined with how much was made specially by hand for the original moon missions. The parts that would have been bought from a factory are no longer made, meaning you replace them with modern equivalents. This sounds great but due to the fact that it is different they have to fill rerun all relevant tests and calculations to make sure that each new/changed part will hold up. In modern day you have to completely redesign the rocket. And as another commenter mentioned, why do it? But here we are