There is some truth in what you say. Airport security is and will always be not perfect, as it is not possible to realize total security, as long as objects (people and luggage) are allowed to get on board. However, real police officers would be better at the job. They are carefully selected and get years of training before they are policemen (maybe this is different in the US, I don't know.). They are better paid (even though I think they do not get enough for the task they have to perform) and they cannot be pushed around like those rent-a-cop people at the airport. Their only obligation is to do their job right.
On the other side, the private companies try to make a fortune and they must compete with each other. Therefore, they try to become cheaper. In that business this is only possible by buying cheaper equipment, but there are limits to that as minimal standards are applied in that area. Therefore, the only option is in pay and training. The result is obvious.
If you pay peanuts you get monkey.