In addition to technology loss mentioned in other replies, reproducing Apollo designs without major redesigns is not a viable option for simple cost reasons: no NASA program in the foreseeable future will have anything near Apollo's budget, and so any new moon lander will have to be much cheaper.
Given the advances in control systems, simulation, and materials I think this is very doable, but it's work.
NASA's budget during the Apollo era peacked in 1966 around 4.5 billion. That's about 35 billion in today's dollars. That's a fair bit more than the actual NASA budget of 23.5 billion but it's not all that far off.
The money is there. The challenges are mostly, as other's have noted, large changes in safety requirements and general execution challenges for large scale projects in today's America.
During Apollo a majority of that budget was dedicated to Apollo. Modern NASA maintains the ISS, multiple observatories, satellites, the DSN, and dozens of similar projects, budget alone does not paint a clear picture
Ya that's true. Apollo was 60-70% of NASA's budget for close to a decade. There's a lot more going on now. If anyone is curios about the details, there's a good summary here:
Given the advances in control systems, simulation, and materials I think this is very doable, but it's work.