How could Jenova have made it herself? She never exhibits that sort of strength, even if she did, she did it by crashing from the sky.
What power does the planet have? Well the ability to create giant powerful monsters for a start.
I realise you are in awe of the power of Holy, so let's look at this another way. Holy can stop meteor, Holy can wipe out all bad on the planet selectively. Then what the hell are the weapons for???
For hopefully this time, the last time, HOLY AND LIFESTREAM ARE PART OF THE PLANET. They aren't separate entities that defend it.
