Because even normal explosives degenerate over time, and fissile material in nuclear devices is even worse about it - remember that unstable elements are ongoin

Because even normal explosives degenerate over time, and fissile material in nuclear devices is even worse about it - remember that unstable elements are ongoing constant fission events, critical mass is just one where they trigger each others' fission fast enough for runaway process. So in order to verify that the weapons are still useful and won't fail in random ways, you have to test them. Which either involves actually exploding them (banned by various treaties that have enough weight that even USA doesn't break them), or numerical simulations.