> How hard can it be to hook them up to AI that is instructed to maximize throughput? The current system will block 3 directions while having a green light on the 4th which has no traffic.

You don't need AI for that, you just need the system configured properly. Lights near my parents' house have been running cycles adapted to actual traffic for at least two decades, probably three... although using inductive loops in pavement rather than cameras.

There are reasons you might want a short green every so often even with no detected traffic... it would help manage worst case situations for a stuck open detector, etc. But those wasted greens shouldn't be long enough to notice. You might also have situations where locally suboptimal behaviors improves performance or safety elsewhere and that may lead to a better overall system results. I'm not claiming this is the situation in your scenario, but it can explain some negative experiences.

Red lights also regulate speed, although I don't know if they are intentionally de-optimized from green for this reason.