That’s not a proximity sensor though. That’s a CCTV camera with AI detections.
A perimeter sensor is typically just infrared point to point beams. If beam gets broken, alarms are triggered.
It’s also one thing covering the perimeter of a house, it’s another covering a perimeter measured in miles or kilometres.
I’m not saying it can’t be done, it absolutely can but the costs involved are significant to both build, operate and maintain.
These bases were built long before commodity fibre optic networking was a thing, I highly doubt there are suitable ducts running around the perimeter to pull fibre and power through (ethernet isn't going to work due to distances).
Then there is the issue of what cameras do you use. The current leaders in AI cameras are also the Chinese, not sure the MOD would want those. Likewise, it will need to be fully closed circuit, no cloud.
I’m sure we can all come up with a lot of ‘why don’t they just’ form our armchairs but in practice, it’s a lot more difficult and very expensive.