In my experience, people leave their carts in the designated spot, mostly well nested. And a store clerk has to come out for about 5 minutes to bring them back every hour or so.

I don't feel like that amount of labor is a tremendous cost for the supermarket.

I worked at at supermarket when I was young. Even when people left the carts in the best possible locations, it was still way more than 5 minutes per hour that we spent out there. Sometimes 2 of us would spent 10 minutes each per hour, but more likely one of us would be our there for 20-30 minutes per hour, especially during busy times.

And that's when the cart collectors were being quick and not just out there fooling around.

In addition, if things got busy inside and we forgot to collect carts for a while, we'd run out and customers would get upset that they didn't have a cart to use. This was especially likely during rain when no customers would return carts or bring carts on their way in.

Even in places with high employment and minimum wage?

Yes. Essentially every traditional Walmart and Target store in the US does it that way, regardless of high employment and minimum wage.

Walmart as one example has a motorized cart mover. The employee collects and stacks the carts into a line, backed by the motorized unit, and then can just gently guide the whole flock of carts and it moves along with little human effort. It enables the employee to move dozens of carts at a time across the parking lot without very much physical risk of injury.