Maybe Adobe has a monopoly on the .psd extension so Daz can't use it for their program?  But then again if that were the case then technically we wouldn't be able to save our renders in .psd format.

Or Daz could be doing what 3D Max has  done. They want a unique extension that works exclusively in their program only?

Those are my 2 guesses.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi