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.