> low-flying objects and the interception trajectories needed

that is the key. It is much more easy from all the aspects - logistics, cost, agility, etc... - to patrol, discover and intercept from a higher flying drone. The drone can (and will be) added with AI (already some) and can come closer for the AI to work better, to make sure and can abort the attack if there is a mistake, while AI on classic radar guided AA requires expensive optics to do that at distance.

>radar guided

FPV, in visual and IR, cost pennies and available in millions of units from Alibaba, while those military radars cost a lot and not many of them are available.

The radar guided AA is used and works where needed and there not much other options - like for cruise missiles, 850km/h. You can see videos - the window of opportunity is usually short as cruise missile is also relatively low flying.