Why do automatic gates only open for cars and other large objects, but not people ?

Firstly, when we talk about the swing gates, we means the doors with big door body, for some big objects like the cars, but not the commercial doors with small door body for pedestrian. Please know it .

To the swing gates, there are sensors in the ground with different trigger types, some are magnetic loops, some are by weight, others are like a metal detector. In either case, a human wouldn't be enough to trigger it .

The gates are triggered by magnetic loops embeded in the ground surface that sense the mass of the vehicle. This is similar to a loops that operate traffic lights for turn pockets. A pedestrian would need to carry a significant amount of metal to trigger the gate.

To the sensors, they detects the large objects like cars for metallic or plastic bodies, when the sensor founds metallic body it will open automatically. But human beings body and structure are different from cars and large objects, it's also the reason why automatic gates only open for cars and other large objects but not people .