One has to wonder what it would cost to build a Saturn V / Apollo using today’s technology. How much could the entire package be lightened to increase its payload carrying ability. Why start from scratch?
All the Saturn V infrastructure is gone. The research is gone. No one even knows how to make one anymore. It was a very impressive design however, or so they say. Even by today’s standards.
A lot. Computers are much smaller and more capable. Composite materials are much stronger and lighter than the materials available in the 1960s.
Why start from scratch?
The Saturn V was designed to be built with materials and devices available in the 1960s US industrial base. Much of that has been superseded, and no longer exists. Even if we started with a blank CAD file, we would still not be "starting from scratch". The ideas and design principles used in rocket design today have the Saturn V as part of their heritage.