@maxdancona,
That's because the myth has developed over time.
Obviously, most OT books are centuries older than the NT. The oldest OT books are commonly dated to the Babylonian exile, that's be 500 yr BC or something. So they reflect a prior version of the myth, very close to just another god, in which Satan is working for God an
adversary to Man.
In the period between the two 'testaments' (200 yr prior to common era) Jewish religious writings (not canonical) tend to become obsessed with apocalyptic thinking. The world is increasingly seen by many Jews as fundamentally evil and in need of some radical salvation. While messianic prophecies proliferate, eg in the Qumran sect, Satan becomes far more that just the "adversary" he was in the past...
Jesus himself calls him by his new title in the Gospel: the
Prince of this World. This is the NT version, a view echoed by the gnostic idea of the demiurge: God created the world but for some reason, Satan now rules it as an
usurper. Hence God will wage an apocalyptic war against the demons at some point, to redeem his creation, unshering in the Kingdom of God. Kaboom! And our job is to get ready for it.
Evidently, nothing of the sort happened for the past 2000 years... COVID-19 notwithstanding, the end of days are not upon us quite yet. So this idea Jesus had of Satan as an
usurper of God and prince of our world had to evolve, and branch out as history and society required. IOW, rare are the Christian who think nowadays that everything in this world is evil and ruled by Satan (=the NT version). Most nowadays would think that Satan rules
hell, rather than earth, and occasionally roams earth to fish for souls.
So the myth keeps morphing. To me, these are just new avatars for an old myth...