A reject actuation has to fire before the part leaves the station. An interlock has to trip before someone reaches the machine. Neither can depend on a network round trip, which rules out cloud inference regardless of how much compute the cloud has.
The second constraint is data volume and privacy. Streaming continuous video from twenty cameras is expensive in bandwidth, fragile when the link degrades, and awkward under most data-protection regimes. Running inference locally means only events and metadata leave the site — and in many configurations, no imagery leaves at all.
Sizing the node is the engineering. Worst-case latency, not average, has to fit inside the cycle time the line allows, and that is measured on the actual hardware rather than estimated from a model's operation count.