It doesn't make it better, large gaps in the line extend the amount of space the line needs in the facility. During busy hours that would extend the line far out into areas needed for other things.

It's only the same thing for the person leaving the gap, due to the fact that there's ropes preventing people from getting between the person standing there and the person in front of them.

Also, if the gap gets big enough that the time spent gathering up your luggage and moving to the front of the line causes delays, those delays will add up during busy times.

It's a game of chicken at that point, and if everyone acted like that, it would significantly increase wait times.