Is there a plugin for this? Tiled preview

[Ofnuts]

Because someone wrote them. Remember, Gimp is volunteer work. For something to be added it needs:

• Someone who understands the problem and has an idea about how to solve it
  
• Has enough time/incentive
  
• Has enough coding skills
  

Of course, the core developer has a say  bout the inclusion of the resulting code in the Gimp code base, but many features are implemented as plugins, so they don't need to be part of the Gimp codebase.

AFAIK symmetry painting is a bit special, because the author (Jehan Pages, otherwise a regular contributor) set up a Patreon account to allow people to subsidize him to give him more time to implement this feature.