Probably... because their only competition is a race created by a divine being to serve them?
You can't evolve to become strong without legit adversity. ||P It's frankly a wonder humanity is as advanced as it is in Pokemon.
But really, do you have any idea how dangerous and complicated pipe bombs are? I mean, /honestly/. Why would they ever feel the need to go through the trouble of making something like that in the first place, let alone building it when they could just... y'know. Do something easier and less dangerous.
Hmm... you do raise a really good point. It could end up being more of a liability than an asset to them, especially against a fire-type spirit wielder. One well-placed Ember, & boom, you've got a bunch of limbless acrobats flying every which way.
Okay, how about pipe bombs? I'm not really sure if they've still got remote detonation technology, however...
Sort of, I guess. It would be very hard to amass enough gunpowder to justify much of a keg, but you could do it. It probably wouldn't even be as effective as a stick of dynamite or a landmine and would be pretty difficult to use even on humans, let alone spirit wielders.