The ASVAB tests for aptitude, hence the name, not the ability to step into a job without training. Looks up practice questions for the Mechanical Comprehension portion to see the kinds of questions that might suggest someone could be a good mechanic.

The U.S. army absolutely does provide training to its mechanics, and does not assume people know jack shit coming in. I scored high enough on the MM portion of the ASVAB to be one and don’t don’t know a damn thing about fixing vehicles.