In the Heroes of Olympus series by Rick Riordan, the ancient lands (Rome and Greece) are a lot more dangerous than America. I would think that the monsters from the ancient lands would have moved along with the gods and others mythological creatures. There are a lot monsters who did move to America, but why didn't they all move. Why is the ancient land way more dangerous than America?
It's not explained very clearly in the book (Mark of Athena is where the concept is introduced, and the other 2 books of Heroes of Olympus simply reinforce the idea that it's super dangerous AND forbidden by Roman camp laws):
Reyna plucked an apple from a passing fruit tray. She turned it in her fingers, studying the dark red surface. “You propose an expedition to Greece in your warship. You do realize that the ancient lands—and the Mare Nostrum—are dangerous?”
“Mary who?” Leo asked.
“Mare Nostrum,” Jason explained. “Our Sea. It’s what the Ancient Romans called the Mediterranean.”
Reyna nodded. “The territory that was once the Roman Empire is not only the birthplace of the gods. It’s also the ancestral home of the monsters, Titans and giants…and worse things. As dangerous as travel is for demigods here in America, there it would be ten times worse.”
Presumably, being the ancestral home would make things worse in 2 ways:
Monsters are stronger there, and there are more of them
Percy hoped the ancient lands wouldn’t be as bad as they’d heard. But it was almost like a commercial: You’ll notice the difference immediately! Several times an hour, something attacked the ship. A flock of flesh-eating Stymphalian birds swooped out of the night sky, and Festus torched them. Storm spirits swirled around the mast, and Jason blasted them with lightning. While Coach Hedge was having dinner on the foredeck, a wild pegasus appeared from nowhere, stampeded over the coach’s enchiladas, and flew off again, leaving cheesy hoof prints all across the deck.
One reason why the monsters are stronger is because many monsters are children of Gaeia, and she said she is strongest in Ancient Lands.
There are some monsters who didn't follow the Western Civilization to America and stayed, meaning they would face those monsters as well - or evil demigods (as random example, Chysaor).
I also reviewed nearly all of his interviews pertaining to HoO series, and none asked this question.
The gods moved to America due to the change of dominance. Nowadays, they reside in the U.S because it is one of the most successful countries in the world. Monsters also immigrated here. But, they as like the gods, are most powerful where they were born. Gaea and Ouranos were also created there by primordial Chaos. The monsters have more power there, especially since Olympus moved with the gods.
Presumably, the monsters now have more control because their "home" is Tartarus which is under the crust of the Earth. Since it is a big pit, Tartarus would stretch throughout the world, just under it. Not having to change much, monsters would have better control over ancestral lands, making them much more dangerous, even though monsters spawn regularly all over the U.S and Canada.
Basically, because gods have followed the Western Civilization, there are presumably less and over time, monsters have gained more or all power there because their home is located underneath the surface of the Earth, giving them the entire world to spawn. Most spawn in America with the rest of the Western Civilization. In addition to that, some monsters do not venture into North America, i.e Chysaor, and Sciron, making them hard to prepare for as they have never been faced before.
Reyna even stated herself that no Roman would dare go to the ancient lands due to the massive power and amount of monsters there. She also said that:
"It’s also the ancestral home of the monsters, Titans, and giants…and worse things. As dangerous as travel is for demigods here in America, there it would be ten times worse."
These 'worse things' can be the primordial gods. Where Gaea is strongest, Tartarus's heart is, and the final resting place of Ouranos, Greece, and Rome are filled with primordial gods, dead or alive.