Why does everybody think this?
"After the mayor and any other player characters in town have had their fortune told by Katrina a cumulative total of 20 times, she will suggest setting up a permanent shop in the empty building on Main Street (next door to the Dream Suite and behind the photobooth) as a public work project for 340,000 bells. The player does not have to have their fortune told in their town; they are able to visit another player's town to have their fortune told and it will count towards being able to build her shop on their main street."
https://animalcrossing.fandom.com/wiki/Fortune_Shop
I got about half of my fortunes told in other towns before she set up shop, and I kept a rough count