Naval reactors are different, their scale is two orders of magnitude lower. One could make other safety guarantees at that scale. They also use enriched uranium which means no refuelling is needed during the service life of the reactor. SMRs can also make some of these safety guarantees.
> Given the (relatively) small size of these reactors, why can't they put 100 of them on site at a NPS?
That's what companies developing SMRs aim to do. For instance NuScale has a design using up to 12 modules of 77 MWe each in a separate stainless steel lined concrete pools of water. The modules are quite innovative and incorporates passive safety features such as natural circulation, redundant passive decay heat removal, gravity driven safety systems.
