Why is the salary of photography assistants generally so low?

Because photography is highly dependent on the artistic level, that is to say, the level of photographic works depends entirely on the photographer himself, and the work of others is only auxiliary and dispensable. To put it bluntly, it's like coolies on construction sites, serving in restaurants, and washing cars in 4S stores. These jobs have little technical content, so the salary is very low. As for your "no shortage of threshold requirements", it is because there are more people who want to do this kind of work, so the studio has a choice and the salary is low. There are fewer people who want to do this job, and the studio has less choice, so the salary has gone up. This is the law of the market, there is no way.