Where does the concept and parameters of a mashgiach temidi come from? Is it in Gemara or Shulchan Aruch? Is it just a standard that was adopted?
It is correct that yotzei venichnas is adequate for many situations, if all the details of yotzei venichnas are observed. In most restaurant and factory settings, they are not observed, so it does not work. And often the yotzei venichnas turns into too much yotzei and too little nichnas, so it is better to have the mashgiach know that he has to be there all the time.
In addition, there are extra sensitive situations where things could go wrong in an instant, such as when there are non-Jewish cooks regarding bishul akum, or when things can be added from the outside. As an example, there are certain medical situations where constant care and monitoring is needed, such as in ICU. Will the family of the patient agree that the nurses leave whenever they want, and just do spot checking in the patient? No! Because at any moment something can happen. It is the same with kashrus, which is the food that we are putting into our bodies and feeding our neshomos. We don’t want to rely on the fact that it will work out, when the situation is such that in a second, the gentile cook can do something that no one will notice, especially if he is not afraid, because there is very little chance that what he is doing will be noticed.