Map conversion mods take an entire city from another game and put it inside GTA V. They are years of work. Most never finish. Some get close enough that they reshape what's possible.

Major projects

Liberty City for V - the GTA IV city. Vice City Remastered - the 2002 city. Real-life city projects (NYC, LA, various others).

Each is a multi-year community effort. Each ships in fragments. The 'finished' status is rarely simple.

Why most don't ship cleanly

Scope. Years of work for a small team. Burnout. Tooling changes that break in-progress work. Legal pressure occasionally - bringing copyrighted assets across borders is touchy.

What ships is usually meaningful even when incomplete. The Liberty City mod is famous in unfinished form.