Hah... wow, if I caught a box of garbage bags in my place that didn't have draw-strings, I'd sh
it a walrus. It's all about speed, ease and efficiency. I don't leave trash lying around in my apartment long enough to rot, so the little gap is negligible at best. In scenarios where I need to throw away something that stinks quick, like meat or shrimp peelings, they go in ziplock bags first if it's a fresh bag that won't be taken out for a couple days.
I honestly don't know why people don't do that by default.
Garbage cans really don't have to stink. They just get the bad rap because the people filling them are horrible human beings.