Ultimately this is an economic argument. With about $10k you can buy a few dozen machine vision cameras [I would say "webcams" if I knew less about webcam compression], a PC with mobile modem, and a self-contained solar power supply. For $10M you can buy a thousand of these. A thousand of these evenly distributed within a few hundred kilometers of your country's borders are going to add dramatic capability to your air defense network compared to going from 5x $10M radar systems to 6x $10M radar systems. The key bit is that $10k costs far, far less than a single strike missile to take out one of these optical detection sites.

A full Patriot battery is $1100M, with the durable radar and command components being more like $400M.

Could the advantage be that a military monitoring installation of cameras is less detectable than a high-frequency radar monitoring installation? My understanding is that active radars are easily detectable, but perhaps cameras could be less so and that advantage could allow them to be deployed closer and might require less protection themselves.