Map conversion mods import an entire city from a different game into GTA V. These are years of someone's life. Most are never completed. A few come close enough to change the landscape.
Major projects
Liberty City for V - GTA IV's city. Vice City Remastered - the city from the 2002 game. Real-life city projects (NYC, LA and more).
Each is a multi-year community project. Each ships in pieces. 'Complete' is seldom a clean label.
Why most don't ship cleanly
Scope. Years of effort for just a few people. Burnout. Tooling updates that invalidate work-in-progress. The occasional legal threat - moving copyrighted assets across borders can be tricky.
What ships usually matters, even when it's incomplete. The Liberty City mod is legendary in its unfinished form.