Rowan did not fully understand a Primordials' mindset, but he could grasp a bit of their motivation, and he knew that at the center of it was power or something that was extremely close to it.
As ninth-dimensional entities, they had the right to call everything but themselves mortal because only ninth-dimensional beings were truly immortal. They had transcended the concept of life and death, and such a state as weakness or decay was alien to them; they were essentially perfect.
It was a mystery where perfect beings such as them would still possess the hunger to consume Reality or contain the depraved desires of Chaos. Did Reality corrupt these beings, or were they always like this?
Technically, without reaching the ninth-dimensional level, Rowan could make himself immortal by creating worlds and then seeding them with his Intent.