It's not just reliably sticking the two subcritical halves of fissile material together, but keeping them there via inertia long enough for enough of the mass to go critical before the much more minor reactions blow them apart via melting/vaporizing the nearest surfaces.

If you had to halves of an atomic bomb core and just clacked them together mechanically you'd wind up with a lot of heat and a big old pulse of radiation, and if you were the one holding this device you probably would indeed die. But there would be no nuclear explosion in the sense we think of it as compared to actual functional nuclear weapons. At best you'd wind up with an energy release equivalent to a few pounds of TNT, which would be much easier to replicate with... a few pounds of TNT.

This has been explored to death, e.g. via the Demon Core experiments, where a critical mass of fissile material was brought together via manual means and the end result was the release of enough radiation to kill at least two people (albeit certainly not killing them instantly) but no explosion.