Seems a bit higher than that on average, Dec/Jan were ~8.3%. Faregates are rarely the rate limiter on throughput (buses moreso than others though). Adding more cuts enforcement labor and increases revenue.

There's a not-insignificant capital cost (faregates need electric and networking), and some users cannot be accommodated with faregates at all, necessitating station agents to let them in through an emergency exit or some other means of ingress.

Actually LA metro switched to faregates everywhere a few years ago. Busses still have some hopons though

There are many stations that lack fare gates, it is not universal throughout the system.

Most of the expo line doesn't have gates.

The enforcement you refer to is not universal or uniform in nature.