Usually the reason is either “it doesn’t work” or “it’s cheaper/better to do it another way.” If you leave those out, I think you’re left with things with ethical problems - chemical and biological warfare, and things that can be fatal to experimental subjects or operators (genetically engineered and/or cloned soldiers, kamikaze vehicles).
Not necessarily. For example, there’s Highlift Systems, which is trying to raise the money to build a space elevator. One would think that this would be something NASA would be doing, but the projected cost of the endeavor is some $15 billion, which is more than NASA’s annual budget (some $14 billion). Now Bill Gates probably has that amount of money stuck between the seat cushions of his couch.
It seems to me that most of the folks who’ve tried to be “supervillians” (i.e. Saddam) have been stupid in how they’ve gone about it. NBC weapons are flashy, but any movement in that area’s going to attract a lot of attention. For a few million dollars, one can have a state of the art machine shop. During the day, this shop can manufacture titanium bolts for the aviation industry (they go for about $4K apop, according to some sources) at night, the shop can crank out updated versions of the V-1 and V-2 rockets. (It sounds stupid, at first, but think about it. They’re pretty successful designs, and with modern manufacturing methods, they’d work better than they did for the Nazis, plus you can slap an old laptop and a new GPS unit in 'em for a guidence system, and they’ll be a helluva lot more accurate than the old models.)
[Ernst Blomfeld] “I have a little device I’d like to test on you, Mr Bond. Henchmen! Bring me… the Pain Beam.” [/Ernst Blomfeld]