It's because if a system has fast travel, it's designed to have fast travel, and it represents a larger design philosophy. There is no way a game developer dedicates time and effort to attention to detail and making a world actually feel like a world if 90 percent of players are only ever going to see a few major landmarks and locations.
To use the most extreme examples, compare the open world of a game like Dark Souls, where everything fits together and has attention to detail to the level that people can discover the lore and stories of the setting just through exploration and discovery (to be clear, I'm not saying they should have to, but it's a clear example of a detailed approach), to the open world of Skyrim, where there's this huge world but all of it is just copy paste barring a few unique and interesting locations, and any item or records you find have no real connection or logic to them. They're just thrown there randomly to save time (or even randomly generated) since most players are never going to see them anyway, with even the goddamn tables in dungeons being bookcases moved half into the floor. Skyrim had the way bigger budget, but because they were obsessed with having this big world they just spread their major locations out and filled the rest with copy and paste, since with fast travel most people are only going to see a few of them any way, and those of us who like real exploration got screwed out of a few dozen actually interesting locations for the sake of saying they had hundreds, all of which were actually copies of each other with nothing interesting to them.
