Why are oral tests and exams not common in American educational systems? My family originates from Russia but I only have experience with American academia. My parents constantly describe oral exams whereupon a student is required to choose one of many cards that have a certain amount of questions on them and then after some time preparing be able to answer the question verbally to the professor, who can then ask follow up questions. 

In my mind, this offers numerous benefits; it's much harder to cheat this system as you have to verbalise your answer so copying is not an option, verbalisation is shown to help with memory's retention, because you don't know what topic the teacher/professor will follow up with you have to more deeply to impress the professor, and many more.

The drawbacks seem limited to me; more people standing around waiting to do the exam isn't great, nor is the fact that this will be a bit more subjective to the teacher/professor. 

Is there some specific reason as to why this isn't done?
An oral exam takes me about 30 minutes (including preparation and discussion afterwards). I have about 110 students in my class. Doing an oral exam for that class would take me about 55 hours. This is not the only class I teach. I teach about 5 classes per semester. In order to avoid some of the subjectivity my department requires that an additional staff member is present. So implementing this for my courses would take about 55*5*2=550 person hours. That is just not practical (not to mention that my colleagues won't thank me for the additional work I give them as they have to sit in for these exams).

I know how long an oral exam takes because I do use it sometimes, but for practical reasons I can only do that for a small number of courses and I have to justify why this is necessary to my colleagues.