Question: There is a huge dungeon that has been unexplored until recently it was opened up to the public by the authority, adventurers are encouraged to form party to ensure higher chance of survivability.

There are 18 floors in total and each floor is consisted of a network of tunnels and caves infested with dangerous monsters, every floor will spawn a champion/boss acting as the ruler of that floor.

Going deeper means that adventurers will be encountering even tougher rulers and monster swarms/waves, but why don't all the monster rulers gang up together against the adventurers just like the behavior of the monster swarms/waves?

P.S: I've noticed that the monsters are not hostile to their kind but will attack anything foreign on sight!

Answer: 
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> the monsters are not hostile to their kind but will attack anything foreign on sight!
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If anything foreign is hostile, than even the guy living upstairs is such. Cooperatively acting requires certain brain skills which not all beings have. Take a bee hive: if you are trying to move it so that it's not flooded by water, would the bees spare you from their sting? No, not really.

Like in the fairy tale of the scorpion and the frog, it's their nature.
### Because then they'd have the share the tasty adventurers

Have you ever had this experience: You're seated at a nice restaurant, a neighbouring table has its meal served up, and they call you over and ask for help eating it?

No, me neither.

Your monsters don't think that their dinner can be that much of a challenge. Plus they'd like to get all those shiny things and if they call for help, they'd need to share the loot.