Perhaps no standing water IN your garage, but there has to be at the very least, a film of water (perhaps just caused by the rain, or gutter runoff) on the apron in front of it.
If it's just water directly from the rain, the most I can advise is to improve the seal under your door (perhaps add one of those glue-down strips).
If your gutters are adding water to your apron while it rains (even if there is no standing water, but they're just creating a temporary film of water), then re-routing the runoff might be in order.
We used to get a lot (hundreds) of millipedes in the office stairwell overnight (and some in the lobby that didn't make it that far), and had no clue why.
One day I bent down to pick something up in the lobby, and it hit me. The stairwell has a light on all the time, and no sweep under the door. You couldn't see the light when standing, but from the ground level you couldn't miss the light under the door. They must have been heading towards it. We added a sweep to block the light, and the millipedes stopped entering the building.