You can't just compare things to the Manhattan project. The Manhattan project was large for it's time, but the thing they were doing was ultimately, simple. You can build a nuclear bomb with simply a large enough sphere of enriched U-235 and it'll explode. Which is what the Hiroshima bomb was - a gun type assembly. This is not a complex device.

The relative complexity of projects only ever increases, because if they were simpler we would already have done them. The modern LHC is far more complicated then the Manhattan project. So is ITER. Hell, the US military's logistics chain is more complicated then the Manhattan project.

The fundamental attribution error here is going "look the power to destroy a city was so much cheaper!"