The job market doesn't support certain overqualified people. I gave you an example.
Rationally, you'd think that a cashier that works 3x faster than other cashiers can make 3x the income. Or even 2x.
A plumber could do that, or a woodworker (they might have to start their own company, but they can do it). But a cashier relies on employers and they're just not going to pay a cashier that much. Cashier being an example. There are many support jobs were it just doesn't pay to be good at your job.
These people are technically incentivized to be mediocre. And they often are. Some of these positions allow growth paths, but not all. And just because I'm the world's best cashier, doesn't mean I'm a good store manager.