I mean, the short answer is: Because people don't like feeling frustrated when the work to decipher a location is more than what they'd expect.
I totally understand why the methodology for quests, markers and waypoints changed between morrowind to oblivion and then again in oblivion to skyrim. Not everyone wants to spend a ton of time pouring over maps to figure out where to go next, especially when the reward for doing so is usually just another trinket or bobble. People looking for a passive experience when they're gaming can't be bothered to do that and would sooner drop the game and have a negative opinion of it for it "being too much", opposed to...well, having skyrim's whirlwind success, despite it being a more shallow open world experience.
It reminds me of people talking about Fez and being like "I don't understand why people love this game so much, I got 16 cubes and then put it down because that's credits". Fez is a game that is so amazing because the most engaging puzzles are the ones where you have to toil in order to figure them out. Doing actual research, figuring out clues, taking those clues outside of the game in order glean hints on what to do next. Hell, I don't think people ever figured out the final puzzle in the game, they solved it through sheer brute force. I took notes in that game as I played and it made that experience all the richer as I finally got all 32 cubes (though I obviously had to look up the solution to the final puzzle). But I don't expect everyone to be down for that sort of experience, and as open world games become more and more mainstream, I'd expect the depth of open world games to become thinner and thinner, because people just can't be bothered.
Hell, I just finished Control and my number 1 peeve about that game is how bad the map is and how there just aren't waypoints, even though the benefits of pure exploration don't factor into the various side missions. Often times, "go to central research" isn't nearly enough information and most of the side missions don't have visual queues, like the mold missions, for you to follow, which just leads to a lot of wandering and backtracking and frustration.