"All your divinity has been filtered out. Compared to the lofty deities of the future, you are more 'human' now, or rather, this is your true form."
"They now only possessed the memories and emotions of that thousand-year period, but what truly governed their thoughts and actions was their present self."
"It wasn't their future self possessing their present self, but rather their present self receiving a gift from the future."
This is the in novel explanation of the reason why they’re so childish.