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.
**The monsters are highly territorial and hate each other as much as the adventurers**

You say the monsters won't attack each other, but it may be that the monster lords have merely partitioned up the dungeon to their liking and have a strict agreement for their own minions to not be on each other's turf. Being isolated from the rest of the world is the exact kind of situation that would produce a highly rigid social structure with little challenging the dominant powers, as there are no external forces to disrupt the politics of the dungeon. Like a city that has been divided up between various gangs but with no police. The monsters *would* fight each other if they were on the same floor, but they so religiously stick to their own territories to avoid fights with other monster lord factions that in practice they never see each other.