With respect, you already said you (personally) couldn't identify subject mastery any way other than written tests. So this comment is simply a tautology.
You don't believe you can identify mastery any other way, so you don't identify it any other way, and so you believe that testing is the only way to identify mastery. OK, but this circular logic is broken.
You could show subject mastery by creating anything, or performing anything, or explaining something verbally to someone else, or by earning money in that field. Writing answers to test questions is rarely a relevant skill and more often tests whether you were told about the exam's quirks rather than mastery of the subject.
You don't believe you can identify mastery any other way, so you don't identify it any other way, and so you believe that testing is the only way to identify mastery. OK, but this circular logic is broken.
You could show subject mastery by creating anything, or performing anything, or explaining something verbally to someone else, or by earning money in that field. Writing answers to test questions is rarely a relevant skill and more often tests whether you were told about the exam's quirks rather than mastery of the subject.