One of the design constraints of a bomb weapon is that it should not go off before you want it to explode.
Another constraint is that you have to be able to get it to the target area while someone is actively trying to stop you from doing it.
Finally, a weapon has to direct its energy such that it destroys the intended target, rather than dissipating most of it uselessly, or harming your own assets.
That's why the physicists didn't just say to refine some plutonium and lob it at the enemy.
There have been 60 criticality accidents, with 14 fatalities. None of these resulted in an explosion (excluding steam), but that does not mean that a criticality accident cannot cause an explosion. A lower critical mass will produce more "flash" than "bang". It can still make a boom. The pow from a cherry bomb is as much an explosion as that from a 500-lb military munition.
A failed reactor will not destroy any cities, like the first nuclear weapons. But it could destroy a back yard, and render the surrounding neighborhood uninhabitable for many years.