3) WHY?

Well what is different between the OT and the NT? What does the NT have that the OT doesn’t have? The answer evidently is Jesus. So the reason we find demons in the synoptics (but not in the OT, nor in John) is the same reason why we find a disobedient devil in the NT (completely different from the obedient diabolos in the Septuagint) Jesus made them; Jesus came to bring the darkness to light, expose the darkness. Also, and this goes back to why Jesus has his disciples cast out demons and then says he sees it as Satan falling from heaven in Luke 10:17-18, because just as Christ’s parable about having been tempted for 40 days by a character from Zechariah’s third night vision was to teach the disciples how he could keep sin out, the healings are also done in this way to teach the disciples (and us) the way Christ casts physical and mental evils out. The purpose of these acted parables is not to teach Christians that exorcism of the immortal souls of drowned giants is a better treatement than anticonvulsants like Topiramate and Fosphenytoin for epilepsy, but to teach Christians that Christ has power over the fundamental cause of illness, and death – sin.

So what is the definition of a demon in the synoptics? A demon in the synoptics is a supernatural evil of popular belief being used in an acted parable to show that just as God had power over Baal Zebub and gods which didn’t exist (and could neither heal nor cure in in 2 Kings 1:2-3, 6, 16 where King Ahaziah of Kingdom of Israel (Samaria), appeals to Baal Zebub who doesn’t exist), Christ has power over Beelzebub who still doesn’t exist – including illness beyond man’s knowledge – schizophrenia etc. but ultimately the lesson of the demon-language of the synoptics is to teach us that Christ has power over the greatest demon, the devil of personal sin-habits, and “through death he might destroy him that hath the power of death, that is the devil” (Heb. 2:14).

Likewise the same definition is true for Gentile demons; there is no question about what the demon which possess the girl in Philippi is, as Luke says ἔχουσαν πνεῦμα Πύθωνος = having a spirit of the Greek god Python. Because she is a Gentile girl, outside Palestine, she is infected by (polytheistic) pagan gods, rather than (monotheistic) Jewish demons.

So that is what the text is. A demon is a demon is a demon.

Does that answer your question? Or is your question, why do those churches who do not believe in fallen angels and evil spirits read these verses in the 3 synoptics as only describing the beliefs of the day? If that is your question then there would be 3 main answers:

1) the 3 synoptic accounts were written to provide journalistic accounts of Jesus and the disciples miracles. They aren’t medical-scientific descriptions, any more than Jesus rebuking the fever (Luke 4:39), fevers don’t have ears. They also aren’t demon-fighting “spiritual warfare” texts such as can be found in the Testament of Solomon, Jewish magical papyri, or the shelves of modern Christian bookstores. Or indeed the demon stories of the Talmud, or rabbinical explanations such as Rashi states that kordiakos is the name of the demon which rules in a drunk person (while to Maimonides kordiakos is simply alcoholism).

2) “demons” are only a subcategory, appendix, of the way the NT treats the devil – with 70+ references. Jewish readers with a strong foundation in monotheism and knowing that God causes illness, death, even darkness and “evil” (disaster Is.45:7) would know that demons (in the OT pagan gods) were just superstition. They would also know that the only two occurences of “Satan” in the OT occur in poetic books – the prologue to Job and Zechariah’s vision of Jeshua the priest’s ghost and Satan in the throneroom, while his grandson and greatgrandson sully his name with intermarriage and bribery on earth below. So those with a strictly monotheistic (not dualistic) ground had a different starting point about supernatural evil than the Pharisees, the schizophrenics, epileptics, and blind of the 3 synoptic Gospels.