Most modern garage door window frames have seals and there are seals around the door itself to deter the infiltration of wind driven rain.  However,  windows, sections and the bottom seal of the garage door may not be 100% leak-proof.
Because a garage door is not permanently fixed in position but moves up and down and is subject to wind loads, perimeter window seals can be breached by vibration of the door or deflection of the door in the closed or open position.