For the obvious reason that whatever level you intend to play to, you'll need NPCs that are 4-8 levels higher than that to serve as villains for the PCs at that level.
Do you? I'm not sure what response you're replying to, but I disagree that you need to have foes more powerful than a PC in order to serve as villains.
Lex Luthor and Superman. Can Lex take Superman in a fight? Nope. Superman is ridiculously more powerful ... but they've told thousands of Luthor v Superman stories. How?
The big problem with high level games is that many people
do not run high level games. They run low level games using high level mechanics. They get frustrated that they can't run a murder mystery when the PCs can just ask a God who did it. They try to run a dungeon delve when the PCs can teleport to the end of it. They try to beat the PCs down with a pure melee brute when the PCs can just use magic to keep the brute at a distance. The DMs are not looking at the PCs as the high level incredibly powerful beasts that they are ... instead the DM think of the PCs the same way they think of 6th level PCs.
How can the game be fun without being able to provide mysteries? You can have mysteries at high level - but they're not a 'Who Dun It' mystery, they're a "What will happen" mystery. Spells can tell PCs what the DM thinks will happen if they attempt something, but in the end there is no spell that guarantees anything. So, instead of the PCs repeating the same mysteries they did at lower levels, the PCs get to skip to the ramifications of a mystery they can solve with a simple spell ... and those ramifications are very different when you go from solving the Murder of a Town Mayor to solving the Murder of the King.
And being able to scrye and teleport past an entire dungeon seems like it means that PCs never have to adventure. That isn't true, either. Teleportation allows PCs to skip past connective tissue, but they still have to go to the places of focal interest. If the MacGuffins are in 5 rooms, the PCs need to deal with those 5 rooms ... and if they skipped over the rooms next to the MacGuffin, then those rooms might provide reinforcements in the MacGuffin room. The dungeon still works ... you just think of it differently and maybe don't need to build so much middle 'fluff' for many high level parties, even if that fluff was the best part of lower level play. It just isn't at higher levels, usually.
So why do we have things like the Tarrasque in D&D if you're not supposed to do a melee brawl at high levels? Read the text beneath the combat box. It talks about it eating towns. It talks about it obliterating everything in its path. It is a Kaiju. While the PCs can fight it, the question will usually not be, "Can the PCs beat it?", but "Can the PCs beat it before it destroys X?"
High level DMing and playing requries a different skill set. It is worth learning - because it can be really fun.