It would be a waste to transform...using up that energy for the next level. They would be better off staying in base form an using that energy in base form...they could reserve so much more energy/power.
Beerus an Whis don't seem to power up, they are in a peak state of god tier and is why they are limited as were Goku an Vegeta can go beyoung that god tier because their power will continue to rise with time and training.
I think Whis states that somewhere in one of the new films.
SSJ God should not exist, it is just something that seems to catch the fans because of Beerus statement of SSJ God. The transformation we see is a saiyan who's pure as stated in battle of gods. It doesn't mean it is a SSJ form, it's just the appearance of a level above SSJ.