FWIW, McVeigh got a lot of the technical details right, including many non-obvious ones. That was a sophisticated attempt by someone that actually knew what they were doing. It goes a long way toward explaining why that particular bombing was so effective.
That said, plenty of extremely smart people assemble bombs in their garage for fun. It is almost a rite of passage, at least in the US. The fact that historically you could just buy the common stuff incentivized smart people to attempt more technically difficult things for bragging rights. Most people have no concept for how available legal high explosives are in the US, even after 9/11 made it a bit more difficult.
