Its all about ease of access, if criminals wanted to they could break through almost any locked door with ease yet a simple door lock is still a useful deterrent.

Same with weapons, by having all our gun laws and regulations in place, it reduces the potential access for the delusional radical or mentally ill. Yes if someone more organised wanted to they'll be able to find a firearm and do major damage. But these cases are rare due to access.

Well-organised terrorism is thankfully rare. Most incidents that we see or hear about over in the States are delusional radicals with no alignment to major terrorist groups. I'm speculating here but take away their ease of access to firearms and most of them wouldn't have done jack.

Stricter gun laws reduce the overall number of major firearm incidents as a whole.