Random Encounter.
Shiny hunters do it because it’s similar to the Safari Zone, except the Pokémon can’t flee, and you can get cool bugs during the Catching Contest.
That’s just about it; it’s a challenge made up by shiny hunters where you just, random encounter and hope you catch the shiny you find.
Source: Experience and Dallas (shiny hunter on YT; was talking about it on stream a few nights ago)
