….nor for something even as simple as a pencil:There is a lot that needs to go right to get a nuclear detonation. Sure you have the reaction itself. But the means to get that to happen occur through quite a bit of mechanical means. So Mechanical Engineers would need to do their part. I'm sure structural engineers do a ton of work with the "delivery" system as well.
A nuke is one of the most complex devices mankind has ever built. It's not just one or three people that have their work in the kitchen with this.
https://en.wikisource.org/wiki/I,_Pencil