A god's descent happened occasionally, not often.
After all, the main plane inhibited gods, and if they descended with their real body, it would be easy to meet their demise if they encountered those with ill intentions.
For example, the Goddess of Fortune thought that she could beat everything if she descended with her true form.
In the end, it turned out she couldn't even beat Mordenkainen.
Instead, she went back to the divine realm and reincarnated.
There was another bunch of evil gods that got killed by Melf.
And some weak gods that came to the main plane and even disappeared.
They couldn't even cause a ripple.
This was why few gods dared to come to the main plane with their real bodies.
Most of the methods used were those like Mystra's, who came over with a divine shadow, and even if the shadow was broken, it would only cost a little bit of divine power.