Search radars have to handle a lot of noise and signals from a number of sources. The radar systems need to be programmed to understand returns from different t

Search radars have to handle a lot of noise and signals from a number of sources. The radar systems need to be programmed to understand returns from different target types and their operators need to be trained to understand the readings. My point is based on the Russian navy's ignominious history it's unlikely their radars and operators have adequate capability to detect any aircraft or weapons that didn't exist in the 1970s when they were first designed. Additionally even if drones were detected the Moskva may not have been equipped with heat seeking munitions. Even if they had them, a drone at stand-off distance can easily be out of range of their targeting. The drones also only need to occupy the ship's attention long enough to distract it from the incoming anti-ship missiles. Even if the drones are shot down it's a small price to pay to disable or sink the Russia's biggest naval asset in the Black Sea. Finding things even in calm seas is hard. Aiming at those things is harder. Hitting those things with a fast moving thing is harder still. Poor equipment increases those difficulties as does poor crew capability. All of those combined makes for single use submarines.