Are you new around here - that's not how it works a lot of the time
It's easy to forget. You're coming and going, someone is watching the rug rat, you've got your hands full and others are coming and going - it's easy to leave it open because someone else is about to go through it, only they don't and you don't notice. Or if, like us, it's only occasional use for the grand kids, then it can be hard to get into the habit of shutting it.
Our nearly 3 year old already knows how to open our gate - she just isn't strong enough, which the rest of use almost share with her
Some of the other gates her parents have used, she learned to open ages ago - often just by brute force.