According to The Scare Factor's haunted house directory, as of August 2021 there are 2,239 haunted attractions in the United States. Outside of the U.S. there are only a handful of haunts, with Canada having the most. Canada has 51, all of Europe has 11, Australia has 2, and Mexico and New Zealand each have 1. Although not listed on Scare Factor as of this publish date, Japan also has several haunts. That means about 99% of all haunts in the world exist in North America. So why is that?
The origins of haunted houses and horror-themed attractions obviously goes back to the holiday of Halloween, which is when most haunted attractions operate. So let's start there.
All Saint's Day (or "All Hallows' Day") started as a Christian tradition that began in the medieval times as a day of prayer and remembrance for saints and martyrs. In the 9th century, Pope Gregory IV formally declared November 1st as the Feast of All Saints. The name All Hallows' Day comes from the old English word "hallowed" which means "holy".
On All Hallows' Eve (October 31), worshippers would fast in order to prepare for the feast day. Some Hallows' Eve traditions likely also originated from an ancient Celtic festival that celebrated the end of the harvest and beginning of the dark part of the year. Ancient Celts would offer sacrifices and pay homage to the dead, dressing as animals and mythological monsters.
Hallowe'en (from Hallows' Eve) did not become a holiday until the mid 1800s when a large number of Catholic Irish immigrants came to the United States during the Great Irish Potato Famine. It became more recognized and commercialized in the early 1900s before the Great Depression and often associated with mischief after sunset. During the 1930s, Halloween-themed haunted houses first started to show up in the United States, capitalizing on the ghoulish history of the holiday.
Fast forward to the present and dozens of other, mainly English speaking, countries now celebrate Halloween to some extent. This is attributed to growing western influence in these countries. Even so, most other countries do not embrace Halloween in the same way the United States does and it is still widely recognized as an American holiday.