Why are oral tests and exams not common in American educational systems?
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.