While it's true that all gods can it has been historically show that few gods do.
Could you elaborate on what you consider "historically shown"? This is not a taunt, it is a question for understanding, because....
My experience has been that "gods" tend to show up in an astonishing amount of forms different from their own. The same god appearing as a dwarf to dwarves and human to humans, or Bahamut appearing human, etc etc.
They all seemed to have the trait of "appear to the mortal as needed" that they consistently use.
Perhaps my thoughts come from gaming and fantasy books, after all Zeus never changed up, but on the other hand, Loki did.
Interested in your thoughts...