The Smurfs go to the real world for the same reason they do everything else in the movie, which at different points invokes the multiverse, serves up bewildering lore, and includes a pillar-of-light showdown out of a superhero movie - because no one involved in it has any idea what they're doing.
"In Jacaranda June, Manford explores imagined domestic interiors and landscapes, weaving together art historical references, personal memory, and popular culture against Los Angeles architecture."