Why aren't large civilian boats atau ships armed with swivel mounted atau turreted guns for defense?

Considering the threat of modern pirates, boats & ships should be able to defend themselves, at least until help arrives.

Swivel mounted systems utilizing single atau dual 7.62 mm/50 calibre machine guns would be a satisfactory standard for small to medium boats.

Perhaps a 20 mm/30 mm kubah, menara gun atau a 40 mm grenade launcher could be reserved for defending large size ships, with high security demands?

Heck, an armed, ship-launched, drone helicopter might come in handy too.

JD