While it is an interesting question regarding how Rajaat interacted (or didn’t interact) with the Rhul-thaun.
But if Rajaat had access to lifeshaping, why bother figuring out magic in the first place?
If he had a massive source of lifeshaping power hidden, why invent defiling?
Yes, Rajaat had powerful psionics, but it seems to me his main schtick had to be magic in order for the entire back story to hold together.
I think the answer is that lifeshaping isn’t very powerful. When it is blended with magic and psionics, it becomes potent.
This. Although lifeshaping is a phenomonally powerful tool, you don’t have limited uses per day and can breed new creations to provide all manner of benefits. But lifeshaping can’t allow a man to teleport across continents, level mountains, or do any of the insane nonsense you can manage with Wish (if at great personal peril).
Given how psionically advanced the Green Age was, I don’t think lifeshaped weapons and monsters would’ve been enough to accomplish the Cleansing Wars. Defiling was a much greater force multiplier and even then the Cleansing Wars took generations upon generations to pull off.
Others pointed out that lifeshaping is a tool with a limited application, but keep in mind - Rajaat was also a dead-bent genocider. He needed dedicated tools of destruction to carry out his plan. Incidentally, defiling didn’t just destroyed his enemies, it made the land itself uninhabitable. Famine would go great lengths to further his plans.
The more I think about it, the more I wonder whether environmental destruction was a tool of Cleansing Wars or just their side effect. I can’t remember whether this was addressed in the official lore - could someone refresh my memory, please?