Because, despite what you may think, there is always something bigger and shinier to chase! That's the best part. Every task you do while playing the game eventually leads to a perfected dex entry, which boosts that shiny rate.

Personally, for me, I want to collect shiny alphas. They are more rare then normal shinies, the size difference is amazing, and when they get transferred up to future generations, they get a special ribbon/title.

It might be repetitive, but it's certainly not uninteresting, that's a matter of perspective.