Not all universes were created equal.
According to the body cultivation method that his mother originally showed to him, it was impossible to create an entirely new internal space in one's body just by wishing for it really hard.
If imagination enabled any cultivator to become inextinguishable and all-power, then the cosmos would have been crawling with trillions of True Gods by this time!
Everything had a price. Even cultivation could not escape this fundamental rule.
Most life forms did not possess powerful minds and spirits from the start.
Their mental and spiritual energies could not possibly support the creation of a construct as absurd as a miniature internal space!
The creation of exceptionally powerful and reality-defying artistic conceptions imposed such high demands that cultivators needed to borrow external assistance.