The store is going to use the fewest number of employees they can get away with for the expected demand. Store managers can be reprimanded for employees getting overtime (resulting in people being sent home early, and fewer people in the store than originally scheduled), and Advance tracks each employee's individual sales and sets goals for them. "Too many" employees means lower individual sales and higher payroll...those metrics look bad for the store manager even though it would result in happier customers.
I remember when I would go into AAP, AZ, OR, etc. stores in the early 2000s and it always seemed like there were a lot of people working there. Usually most of the computers behind the actual parts counter were used exclusively for looking up parts or billing commercial customers. Once the counter person pulled your part, you took it up to the checkout, and there were one or two people who just rang customers up...no looking up parts.
Now, it's rare to see an AAP store even using the front most counter that used to serve as the checkout only register. There are two or three people at the most simultaneously looking up parts and ringing up customers at the same terminals. With individual metrics being tracked, the old system of looking up parts at the counter then taking them up to the checkout for someone else to ring up no longer works.
In addition to the employees being expected to multitask more and more, looking up parts has become more complicated than it was 20-30-40 years ago. Cars are more complex with more parts, and the counter people these days are mostly not experts and neither are the customers, so there tends to be a lot of confusion. There's a lot of "I need, I think it a solenoid switch or a module thing, I don't know what the man said, it for my Chevy Arcadia, I think it a 09 or a 12." At that point the counter person is totally dumbfounded and the next person in line is going to be there a long time.