I mean, it’s not a very exciting answer, but because that’s how D&D does it, and D&D is the predominant conception of RPGing. To me, the question is ultimately why D&D did it that way. I suppose “because D&D is a wargame” is a valid answer, but then… why are wargames that way?
Well, THAT is easy, because wargames go all the way back to the Ur game, the original Kriegsspiel, which was intended to allow you to play out realistic battles on a tabletop (think basically Chainmail, but written in 1819...). So, from that starting point, where the 'map' was a real genuine military topological map from which the sand table was configured, wargames have almost always had maps! There's a direct line from KS to FKS, which was the practiced version of KS that was used by the Prussian General Staff for officer training. It included a referee with the power to essentially make any ruling, and players who assumed the roles of officers both in the field and at headquarters.
This lead, through some iterations of military training techniques to Wesely, who developed the Braunstein scenario, which he ran with Dave Arneson in which players were given specific character roles. That was elaborated by another guy (who's name I now forget) who created 'Brownstone', which was the same idea translated to the old west, except he introduced one critical new thing, the characters were persistent! All of these still had military dimensions and used maps (in Brownstone I guess it featured fights with Indians and Outlaws, etc. there's not a lot of info online about the details). Arneson then took THAT game and made Blackmoor out of it (critically retaining the persistent characters that you played game after game) and famously grafting on the Chainmail Fantasy Supplement rules to run the fights.
Again, Chainmail, like its great great grandfather KS, depends utterly on maps (or at least setups of tables that act as maps) and because Blackmoor and the C&C Society campaigns that used Chainmail were CAMPAIGNS where the armies moved around countries (this is where The Great Kingdom and such come from which appear in Darlene's Greyhawk map) and you needed campaign maps to play out things like "I'm marching across the Abor Alz with my orc army to get between the Knights of Furyondy and the Elves!" or whatever.
The point being, all wargames use maps (AFAIK, granting there is probably an exception somewhere) and D&D was DEFINITELY a wargame, or at worst a part of/adjunct to a wargame.
Here's an interesting point. The other day I watched a Youtube video of an actual Free Kriegspiel. They didn't call it that, but it was CNAS organized study of a conflict between the US and the PRC over Taiwan. There were two teams, Red representing the Central Military Planning Committee of the PLA and Blue representing the Pentagon/NSC. It had a GM (some PhD from CNAS) and the two teams consisted of a couple of US Congressmen, a couple ex-generals, and some various experts on relevant topics. They had a hex map, units, and everything, though I got the impression that the 'game mechanics' were fairly loose, and they only described each sides actions at a pretty high level. Still, it was absolutely FK! (much refined presumably) and maps were one of its central features.