Some Dungeons & Dragons monsters, like a Hooked Horror work well in any adventure- a worth foe to test your steel and magic against, while other monsters like the Lich ARE the adventure- nothing less than a dedicated adventure or even campaign will do.
From the perspective of the Dungeon Master I have only thrown maybe 3-4 Liches at the party through all my years of DM’ing the game. Kagnon The Putrid Defiler being my main goto Lich. Who so few?
A Lich has the dual-lethality of being both a high level magic user AND a powerful undead with immunities and powers from the transformation itself. Players need to be at an equal high level to even challenge a Lich.
Another point to keep in mind is that a Lich will also have numerous magic items- high level magic items in its possession, and it will be using them against the party also. This jacks up the difficulty level in the Monster Manual which doesn’t account for it. Add to this a Lair, guards, and various proxy minions and dealing with a Lich is more like storming a stronghold- which is why they make such great campaign fodder.
Minions and servants working up front through various legal businesses and enterprises to further the goals of their patron, known or unknown, discovered by the party, leading to the confrontation with the Lich.
A great campaign can be written on this alone…