Because "the wages of sin is death" Rom 6:23 Ezek 18:4 (in fact all of ezek 28) and the doom of the demons written in Ezek 28 "fire consumes them" .. turned to ashes "they will be no more"...
You are not rightly bringing contextual passages to the conversation.
Demons do not have bodies to "burn to ashes", and a body is not the spirit of the kind.
Demons are the "bastard wheat" -Darnel/Tares that have already been doomed to their eternal judgment by God, as in Enoch's book, and will stand for no other judgment but will be gathered by the angels off the earth, bundled up, and cast into the Everlasting Fires [=the Kingdom of Darkness, which is the Second Death] as reported in the Book of Enoch; but they have been granted to walk the earth as disembodied evil spirits on earth, serving the satans, who are evil spirits in heaven, and will do so until their doom is consummated at the end of the great Tribulation -which Tribulation Enoch wrote of as "the second consummation of sin on earth".
Enoch wrote to them, from God the Great Glory's own mouth.
in Enoch 15, The Word is given that the angels in heaven do not marry nor give in marriage, [which Jesus referred to when He told the Sadducees they erred, not knowing "SCRIPTURE" =Enoch's writing, on
that fact; for those who will be resurrected to the Kingdom of God in their bodies of regenerated flesh will be equal with, companions of, and like, the angels in heaven, as only Enoch reported in his book; but the fallen watchers did do that evil transgressing the eternal commandment and fornicated with "alien flesh" by marrying the daughters of Adam in their flesh bodies that the Watchers, who are "flames of fire in heaven", can appear in on earth when they serve God on the earth below. Those fallen watchers took the oath to remain in those bodies and never return to their own estate/domain in heaven, as flames of fire, but to remain on earth and fornicate with daughters of Adam and getting offspring by gene splitting.
The Book of Enoch, Translated by Robert H. Charles, 1912
Enoch 15 And He answered and said to me, and I heard His voice: "Fear not, Enoch, thou righteous man and scribe of righteousness: approach hither and hear my voice. 2And go, say to the Watchers of heaven, who have sent thee to intercede for them: 'You should intercede for men, and not men for you: 3Wherefore have ye left the high, holy, and eternal heaven, and lain with women, and defiled yourselves with the daughters of men and taken to yourselves wives, and done like the children of earth, and begotten giants as your sons? 4And though ye were holy, spiritual, living the eternal life, you have defiled yourselves with the blood of women, and have begotten children with the blood of flesh, and, as the children of men, have lusted after flesh and blood as those also do who die and perish. 5Therefore have I given them wives also that they might impregnate them, and beget children by them, that thus nothing might be wanting to them on earth. 6But you were formerly spiritual, living the eternal life, and immortal for all generations of the world. 7And therefore I have not appointed wives for you; for as for the spiritual ones of the heaven, in heaven is their dwelling."
And then, the doom of their offspring is given to Enoch to write to their fathers who are the fallen watcher angels:
"The Creation of Unclean Spirits
8"And now, the giants, who are produced from the spirits and flesh, shall be called evil spirits upon the earth, and on the earth shall be their dwelling. 9Evil spirits have proceeded from their bodies; because they are born from men and from the holy Watchers is their beginning and primal origin; they shall be evil spirits on earth, and evil spirits shall they be called. 10As for the spirits of heaven, in heaven shall be their dwelling, but as for the spirits of the earth which were born upon the earth, on the earth shall be their dwelling. 11And the spirits of the giants afflict, oppress, destroy, attack, do battle, and work destruction on the earth, and cause trouble: they take no food, but nevertheless hunger and thirst, and cause offences. 12And these spirits shall rise up against the children of men and against the women, because they have proceeded from them." {Top}
16 From the days of the slaughter and destruction and death of the giants, from the souls of whose flesh the spirits, having gone forth, shall destroy without incurring judgement� thus shall they destroy until the day of the consummation, the great judgement in which the age shall be consummated, over the Watchers and the godless, yea, shall be wholly consummated.
2"And now as to the watchers who have sent thee to intercede for them, who had been aforetime in heaven, say to them: 3'You have been in heaven, but all the mysteries had not yet been revealed to you, and you knew worthless ones, and these in the hardness of your hearts you have made known to the women, and through these mysteries women and men work much evil on earth.'
4Say to them therefore: 'You have no peace.'""[/B]