How many enemies, do you suppose, know what LTH even is? Can they make the necessary Arcana check to know that the opaque thing that arrows and spears bounce off of contains enemies? It's easy to assume enemies are prepared for the player's shenanigans, but in reality, the monsters should know as much about the PC's as the PC's know about the monsters- and since 5e lacks a dedicated monster knowledge system, that runs into it's own problems.
Many of them. I don't know about you, but our games generally revolve around something more than a bunch of carrion crawlers eating garbage in a city sewer. There are thieves' guilds, yuan-ti sects, gnoll hordes, orc strongholds, dragons, a myriad of demons, undead (many of which are intelligent), deurgar and drow cities, modrone cumminities, ogres and onis, goblin enclaves, hags, and anything in hell. That is 3/4 of the Monster Manual. All of these could have the ability to know what LTH is. All of these could make life miserable for someone sleeping inside for 8 hours and then getting up to "go adventuring." Heck, half of them, if the DM was playing with any fidelity, would probably be able to wipe out the party the second they came out of the hut.
Some adventures aren't designed to take LTH into account. Or Rope Trick. Or Catnap. Or Galder's Tower. Or House of Cards. Or Alarm. I could go on, but there's any number of tools players are handed to them by the game itself to overcome challenges in the field.
All of the WotC adventures are designed with LTH in mind. It is a literal spell you get at 5th level. Try using it in Storm King's Thunder where some hill giants find out where you are. See what happens. If they don't know where you are, then it is very similar to hiding and getting a long rest. The only difference is you escape getting awoken by a wandering monster. Big deal.
Also, again, if we as DM's have to go to such lengths to make sure the players don't use said tools just to make the game run smoothly, isn't that a huge problem? Why are we forced to fight against the system itself when we want, say, rations to matter in a world with Goodberry or Create Food and Water? When we want to force players to rest on our schedule, when the game literally says to them "if you want to rest, use this!". At what point are we making sure there are enemies immune to radiant damage so Spirit Guardians doesn't wreck every encounter with large numbers of enemies? At what point do we avoid using undead because the Cleric might turn them?
The DM doesn't have to go through any length. If the DM did their job in creating good hooks, a narrative that works for the players, and something other than mindless monsters, then LTH is not an overpowered or game breaking spell.
I believe that getting into an arms race with one's own players isn't healthy for a good game- so why does 5e seem to encourage us to do just that?
There is no arms race. There is a consequence for sitting in a stronghold, or plane of hell, or lich's realm, or cultists' dungeon, or dragon's lair and sleeping for eight hours if you are found. That's all. If the players believe the narrative, they will understand that their opponents are not mindless, but instead, evil and intelligent. Once they believe that, LTH is simply a way to rest in hiding while avoiding wandering monsters. (If you use such things.)