RP1 deposits too much coke in the engines to allow reuse, or even extensive static test firing.
LH2 is stupid dangerous to use, requires physically large rockets due to the low density, and as I mentioned, requires extensive leak detection to be integrated into the design.
Super-chilled methane as used by Starship is "just better".
You really should read through the Saturn V manuals and documentation. NASA put a lot of it up online. Technology has moved on from then.
At the time, cost was no object, so they used approaches that wouldn't be acceptable now. For example, the cooling of the engines used hand-brazed tubing, which is crazy expensive to manufacture and test to the required safety level.
Modern engines simply use CNC machining to cut channels into a solid wall, then a plate is brazed on top. Much simpler to manufacture, much cheaper, much safer.
But CNC machining didn't exist back in the Saturn V days! This is the point. We wouldn't "do it that way" any more, because we have tools that they didn't have. Conversely, they had workers readily available that had skills that are not in demand any more, which would cause a hiring bottleneck if the construction of the rocket was attempted today.
Designs don't exist in a vacuum, in some abstract space of "raw materials go directly into the finished product". Designs are made for the wider landscape of the industrial base and the available engineering skillset.
There is no practical way to take the Saturn V design as-is and manufacture it today. It's possible, just not practical, let alone cost-effective.