To put it simply, it's just a wisp of thought, even if it dies, it doesn't have much impact, nor will it benefit the worlds controlled by the Chaos Demigods.
Jiang Chen has the Prehistoric World cultivators invade the worlds controlled by the Chaos Demigods to plunder their original sources, not to give them benefits.
If the Prehistoric World cultivators were to send their physical bodies to these worlds, then upon their death, their power would integrate into these worlds.
In that case, not only would Jiang Chen's purpose of invasion not be achieved, but it would also strengthen the power of these worlds, which would be somewhat counterproductive.
That is why Jiang Chen has everyone arrive in the worlds controlled by the Chaos Demigods in the form of Divine Thought. This way, if victorious, they can plunder the original sources of these worlds, and if defeated, they won't bring benefits to them, inherently placing them in an invincible position.
