Why are all the policemen on the plane plainclothes but don't wear uniforms?

Everything has advantages and disadvantages. The security check at domestic airports is very strict, and there is basically no threat to those who can get on the plane. Wearing casual clothes can also make passengers feel relaxed. I don't think anyone likes to sit next to them often, which will make them unable to relax. One more thing, if there are terrorists or something, plainclothes police will make him even more afraid to commit crimes because he doesn't know who to watch out for. If he wears a police uniform, his every move will be exposed to terrorists.