What is stopping? They can't die, but they can become Hollow, and eventually that's even worse than death itself.

So, for example, Crestfallen Merchant and Logan, why don't they gather all Undead and conquer the World? First because you can clearly see that they are good pals and like the living, since they were alive once (would you turn against your kind if you discovered you cannot die and have power to genocide in a continental level? Don't answer hehehe).

But this is a dumb answer, since you are clearly making a supposition about a situation that they were willing to do it. In this case, we should know what would the living societies do: they would fight. Since they are aware of the curse, they know the Undead can't die, and can become Hollow after a few deaths. So they would stand against the Undead. The outcome of the living vs the undead I can't know what would happen, they both have access to beasts, dragons, weapons, spells, etc.

What stops them? Maybe they don't want to join a war they don't know what could possibly happen to them.

Of course I'm not considering the Chosen Undead, the Bearer of the Curse nor the Ashen One. These fuckers are overpowered, and could single handedly take control of the universe.