The issue is time. Say it's a YJ-83 clone, so around mach 1.4 terminal velocity (476 m/s). Phalanx tracks at 10km, engages at around 4km. So it has ~8.4 seco

The issue is time. Say it's a YJ-83 clone, so around mach 1.4 terminal velocity (476 m/s). Phalanx tracks at 10km, engages at around 4km. So it has ~8.4 seconds of engagement time to try to destroy the incoming missile. ... And that's a relatively slow modern target. A P-800 gets up to mach 2.4. BrahMos is mach 3? I'd guess the Navy would generally rather keep CIWSs on a hair trigger, and just space their ships further apart in combat situations. And afaik, most of the CIWSs have been removed in favor of RAM, since the gun-limited engagement ranges don't make much sense anymore.