I would think there would be a wee bit of real estate on the side of an aircraft carrier that’s a 1,000’ long.
A CIWS installation is 7 tons by itself, not including ammo. Add to that the tons and tons of scaffolding and decking you’d need to install one on the side of a carrier hull... and then realize that a CIWS cannot shoot straight up. So you stick a bunch of them on a carrier and now have a top heavy carrier that’s still vulnerable to plunging fire and will roll over in a stiff breeze.