Primers are actually better-protected from shock and impact when inserted in a cartridge case - the case surrounds most of the primer and shields it from impacts. Only the back of the primer is exposed, and in order to discharge the cartridge, you need to strike it hard with a narrow/pointy object (pistols hit the primers with the firing pin, which has a diameter smaller than the primer, to set off the cartridge). When the primer is in the open, the explosive mix in the primer cup is open to atmosphere, or, say, being hit by a pen tip or something. Yeah, it almost seems a little counter-intuitive that they're less of a hazard when assembled into a cartridge, but that's just the way it is.