What prevents Orcus from showing up at 1st level and stomping your party flat?
You are not in his area of interest and its a long commute from the Abyss.
Maybe after you blow up a few cults of Orcus. If theres a prophecy alerting that you are a threat, then for the prophecy to be true you would HAVE to be a threat, meaning the very future that predicts those who would be his downfall would arrange for such threats to grow to power. Other sources of information are wide, a Lord of the Nine rules an infinite sized planar layer, owns hundreds of others, and influences even more. They have an infinite sized area to extend near infinite power, and indeed, they are stretched thin.
Maybe we should put out a metric. CR per square mile of control. CR per square mile of influence. The Realms has the former as a large number indeed. If your adventure premise falls inside the area of influence, you might encounter their agents, which is fine. If your premise threatens to intersect their area of control, then they can and WILL turn up. In fact, a lot of them explicitly possess means to monitor their entire area accurately, so not knowing would imply special causation.
No, but Planescape STANDS without the Powers That Be. If you, at one fell swoop, annihilated the Lady, and all the alignment faction leaders, things remain stable, the balance of power remains. The forces of Hell and the Abyss remain matched, with one side using superior organizaton and strategy, while the other boasting numbers and tactics. You can change things, whatever the level, because the conflict extends all the way down to the least lemure. But you change things at the lemure level if you fight lemures, eventually drawing attention from the higher powers and becoming a pivot as each side notices your growing role in the conflict.
Compare this, the Simbul's nation survives against Thay because of Her alone. She literally flies out of her castle with a custom spell that allows simultaneous wands and staves to be activated, and personally vaporises enemy forces. remove her, and the balance of power is nonexistent. The leaders do not reflect the led, nor any economy or strategic arrangements.
Same goes for the evil factions, remove their leader, watch the house of cards crumble. Manshoon, Szass, the list goes on. There are exceptions of course. The areas depend on who develops them.
It's a world full of
retired PCs, because this is exactly what PCs at the end of level 1-30 campaigns used to do, pick out the kingdom they want with all their awesome stuff and settle in. It's a post-game setting, the story is over.