Because videogames and reasons
One Grand Theft Auto fan has taken disparate pieces of information from various findings and sourced them all to create an approximation of the world map of Grand Theft Auto V. This is why we have the "HAHA Weird!" tag. It's an impressive undertaking, so naturally we'll have to compare that map to the original one created for Grand Theft Auto: San Andreas.
The GTA V map is, of course, not approved or confirmed by Rockstar at all, just another example of the passion that runs deep for this game. So deep. Like in the loins and stuff.
Complete Grand Theft Auto V map revealed [Stick Skills]