Tumah why Prohibhited

 Chinuch (159) The wisdom of God is greater than all wisdom and that He only commands a thing for our good and for our great benefit — we have no difficulty or question in all of the prohibition of foods and distancing of impurity, in everything that the benefit to us is not known and graspable by investigation; as we truthfully know, it is all for the good. And do not wonder, my son, about the matters of impurity, if it is very hidden from every creature — as it is possible that impurity injures the soul and makes it a little sick. 

Chinuch (362) The commandment to send the impure out of the camp of the Divine Presence: It is known to the Sages that the matter of impurity weakens the power of the intellect and mixes it up and separates between it and the pure and perfect Elevated Understanding. And it will be separated until it is purified. And it is as it is written regarding the matter of impurity  “and do not become impure with them and be impurified  in them” —The sources of intellect are stupefied by impurity. Hence it is not fitting for a person who is sullied by impurity to be in the holy and pure place, wherein the spirit of God is found And this matter can be compared metaphorically to the palace of a king from which we distance any man that is leprous or disgusting in his body, or even in his clothing. 

Derech HaShem (01:05): And there are surely many different levels about the matter of the powers of evil that we mentioned, and that which is influenced by them. And generally that which is influenced by them is called impurity, darkness, pollution, mundane or that which is similar. And that which is influenced by the shining of His countenance, is called holiness, purity, light, blessing and that which is similar to it

Kli Yakar (Vayikra 21:02) For his kin. The reason for the prohibition of impurity is because of the spirit of impurity that remains attached to the human body. Therefore it is said he shall not become impure for a soul and it does not say “by a soul” because the soul itself is pure and has no aspect of impurity. However, the soul is the cause of impurity because all other living creatures that do not have a rational soul but only a life force — their death is merely the separation of the four elements, and the Angel of Death is not involved with them at all. Therefore, they do not have severe impurity. But the human soul does not depart except through the Angel of Death, who removes it from there against its will,

Mamar HaIkkarim (03)However the truth of the existence of good and evil is that the Master placed holiness and impurity into the world. The explanation of holiness is the existence of closeness to Him and that of impurity is the existence of distance from Him. Holiness is the impact that the Master, effects upon someone who is fitting for it and the influence that descends upon him. And impurity is the distancing that G-d distances Himself, and the absence that He so produces. However the truth is that the Master, created various spiritual powers for this purpose. And darkness and pollution are effected by them, such that in every place that you find this pollution, He will remove His holiness from there and remove His light from there, . And these are called the powers of impurity.