It's rather simple. They pay poorly so they get/retain employees who are unskilled.
From Glassdoor:
Compensation at Geek Squad Reviews from current and former employees reveal: Salaries – Pay is often seen as inadequate for the effort required, with starting rates around $15-16 per hour and maxing out at approximately $21 per hour.
In July I hired a 26 year old as an IT trainee. He has no college degree but he's sharp, polite and his communication skills are impressive for someone of his age. I started him at $ 25 an hour. I expect he will be at $ 30 by the beginning of next year.
You don't get competence for $ 16 to $ 21 per hour.
I'm glad to hear your experience was positive. I don't think that is the norm.