It's not that simple. Even adding additional time doesn't always work. E.g. Some things just can't be done till moments/minutes before having to leave. If you want to do those early you have to leave early as well. Then you arrive early, and then what. So you plan from the other side as well, and that means cutting things close with x% of buffer. Sometimes that buffer isn't enough and it throws everything off. Sometimes you're all set to go, slightly early, and something pops up.

Or, all the planning goes out the window when one of them is wailing or throwing a tantrum, or is so so so eager to show you this wonderful thing they finished drawing for you. Then you have to adapt that wonderfully planned schedule/plan, so your previously-allotted buffer dwindles and next thing you know you're one new-issue away from being late.

But yeah, one of other comments mentioned. There is a difference between being late once in a while vs always.