It seems to be an accepted fact that creating pre-designed adventures for high (15+) level characters is difficult.
I would like to accept that argument but with the caveats that a) "difficult" does not mean "impossible" and b) that difficulty is a technical problem that can be solved and is not a systemic problem.
As such I would like to discuss in a serious way what those technical challenges are and how they can be addressed. What I don't want to do is argue about whether the basic premise is true. Nor do I want to discuss the issues of "fluff" around high level adventures -- that is we won't be talking about whether the 18th level characters would be better off doing something else that adventuring.
Remember, the primary goals are to identify problems and discuss potential solutions.
The first thing to came to my mind is a problem that is true for all published adventures but definitely exacerbated at high levels: the designer does not know the composition of the party.
One solution is to plan the campaign out from level 1/3 up to 20. The last campaign I DMed was built that way. It started at low levels with low-level adventures. Rescue the village from the werewolf. Kill some Zombies. Explore that ruin etc. pp.
While doing so they get involved in the world and exposed to the backstory and the main conflict of the world (this continent was separated from the forgotten realms to quarantine it because of a Lovecraftian entity that was trapped thousand years ago, after it tries to open a hole in reality to let other Lovecraftian things in).
So the stakes kept escalating. From Werewolf threatening a village to an evil IT-like entity killing all the magical children in a city to reassembling the splitted soul of a god to fighting a Kraken that tries to invade a city, getting sparks of divinity and the option to become gods themselves up to find out that the dragon god who you thought was on your side is actually in league with the Lovecraftian monster he is supposed to guard and he is grooming you to become the sacrifice needed to free the monster and open the portal so that the Lovecraftian monsters can colonise this world and reshape it to their liking until they have completely drained it of all life and energy and continuing to the next world.
To create high-level adventures that make sense, you need a world and story that supports that. So from the early levels on you need to build in the 20th Level BBEGs and you need to build in reasons, why they not have taken over the world yet, but become increasingly more dangerous over time.
Like the main trope of high fantasy: the dark Lord who is trapped/lost his power and is bidding his time to get free/get the power back.
But for that all to work, you need the players to want to play safe-the-world campaigns. They need to get invested in the world to play for 20 levels.
And they also need to be okay with not having a choice (because if they don't engage with that adventure, the world will end).