The ones who run out of money.
If you fail your ground exams, you take them again. And again. And again. If you fail your skill test, you can take it again. And again. And again.
Maybe after a third/fourth attempt, the examiner would start to question the person who signed off the 170A, the school as well as the student's abilities.
But there are many who fail to find a job.
Cheers
Whirls