In essence the light pole is designed and rated to be a light pole and not a mast for a sailboat. The oval cutout may or may not be reinforced to take the loads but the assumption is that it is not. Light poles don't have weight issues like boats do and there are also no stability problems. It is probable that the manufacturer finds it easier/cheaper to increase the wall thickness of the tube to compensate for the hole. You could do this also on a boat but the performance would suffer.
As for using the pole base as the step, there would be no problem for the pole but a large one for the deck. There would be a tremendous moment to resist. Imagine the bolts on one side pulling up and the other side pushing down. The bolts being close together will have very high forces. In addition to the bolts pushing up or down there will be a twisting force requiring a large deck beam(s).
If you imagine moving the bolts apart by making the base larger the forces would drop and the twisting moment would be smaller as well. If the bolts are out at the gunnels they would be acting just like shrouds and chain plates. Having a keel step and a deck collar also separates the "bolt" forces. In addition, because of the orientation of the supports, tthe forces in the deck would be horizontal instead of vertical. This can be resisted by the already existent deck material instead of a post and large deck beams.
So, by not having an access hole the entire tube material can resit the stress and the pole can be made lighter. By stepping the mast through the deck on the keel, the deck can be made lighter with less work. Lighter is almost always synonymous with better performance and in this case would be cheaper as well.