In fact, there are a lot of standard designs. There's just not A standard design.
The issue is we build 1 or 2 plants at a time with a given design and by the time those plants are finished (10+ years) new regulations and new standards are in practice (see Gen II vs Gen III vs Gen III+ vs Gen IV reactors).
The good news is that Gen IV reactors, if approved, are much cheaper to build than Gen III/III+. The bad news is nobody wants to build them.
The issue is we build 1 or 2 plants at a time with a given design and by the time those plants are finished (10+ years) new regulations and new standards are in practice (see Gen II vs Gen III vs Gen III+ vs Gen IV reactors).
The good news is that Gen IV reactors, if approved, are much cheaper to build than Gen III/III+. The bad news is nobody wants to build them.