We are told that Moses was the greatest of prophets. Yet we find that he remained in this world and died a natural death which is not the case with Eliyahu who was conveyed into the spiritual realm in a chariot of fire because of the great spiritual heights he attained. Would this not indicate that Eliyahu was on a higher spiritual plane than Moses?
I understand the logic of your question, but to understand my answer, it will help to revisit Eden.
Mystical sources teach that Mankind, Adam and Chava, were created primarily as spiritual beings in bodily form, where their bodies were so illuminated by the light and power of their souls that their bodies were actually more soul than matter.
This is because their five levels of soul – nefesh, ruach, neshama, chaya and yechida – were harmoniously fused within their being in this world and infused and energized it with a spiritual illumination that made them more beings of light than flesh.
When Adam and Chava transgressed, they greatly severed and occluded their spiritual connection such that they simultaneously plummeted and were pressed into materialism where the lower levels of soul – nefesh, ruach and neshama – became rooted in a physically congealed and opaque body. The higher levels of soul – chaya and yechida – remained in the transcendental realm, only tenuously connected to the dislocated levels of soul and body below.
This is the meaning in the Torah of their becoming aware of their nakedness and G‑d’s clothing them with garments of skin. Once their inner light, which initially masked their bodies, dimmed, their transgression became exposed, and G‑d enveloped their crystallizing bodies in human flesh and skin.
From then on, the spiritual and physical planes were greatly separated and the highest level to be achieved in this world was the perfection of neshama. One who approached perfecting the heights of the next level chaya would either have to descend back to the physical, or ascend into the transcendental. Since the congealed, crystallized body cannot contain the light and energy of chaya and yechida it must either retreat and remain physical or ascend and become spiritualized by them, passing over to the other side.
This is what transpired with Eliyahu who was on such a high spiritual plane that once he perfected chaya, this connection could not be maintained in the demoted state of this world and he metaphorically ascended heavenward in a chariot of fire, meaning his physical form was consumed by the spiritual fire of his higher souls and was conveyed into the spiritual realm.
But as high as a spiritual elevation as this was, Moses’ level was even higher. And this is precisely because what happened to Eliyahu did not happen to him.
As did Eliyahu, Moses also attained and perfected the level of chaya. However, unlike anyone other than Adam before the sin, Moses was so elevated that his spiritualized physical form could actually contain chaya in this world without becoming consumed by it. As portended earlier by the bush in which blazed a fire but was not burnt, his body became like a wick upon which rested a heavenly flame and a beacon of light suffusing spiritual luminescence from the transcendental realm into the material.
And this was the reason behind Moses’ beaming with light of an intensity which could not be gazed upon by others. His physical form was literally aglow with the brilliant energy of his higher soul as he regained a spiritual level akin to that of Adam in Eden before his demise. For this reason his “skin” is described as “luminescent”. He was on the threshold between a fallen being of skin and a redeemed being of light. But in the Sanctuary, the portal between the upper and lower worlds, Moses was able to unveil his face, meaning divest himself of occlusion, and truly shine in the presence of G‑d.