[jandrese]

The difference is size and cost. Missiles are large and expensive so ships can carry only a handful of them, and anti-missile defenses are designed based on that assumption. Drones defeat anti-missile defenses by coming in huge swarms that skim the waterline. The drones are also small because they have a relatively small warhead compared to anti-ship missiles.

It's the difference between needing to stop a dozen or so MIG-17 sized hypersonic missiles vs. hundreds of oversized quadcopters that move relatively slowly but chaotically.