Everyone is aware of everything all the time. No need to provide potentially useful information. Nope.

They tend to move faster, so maybe it was full and now isn’t while they waited for a cashier.

Or maybe they just didn’t notice them either? People aren’t very perceptive, especially as they get older and start to lose their vision as well.

Edit, they also seem to think the cashier is obligated to chat with them forever, the people want to talk, cashiers just want a paycheck.