If some of us are questioning un-telegraphed traps, the question we are asking is "what is the point?" Your answer to that question seems to be "because in a real world traps would NOT be telegraphed, so it feels more realistic to have them be a total surprise."However, saying that not all traps should be telegraphed seems to be frowned upon, so I end up in the position of defending when traps should not be telegraphed and only speaking to that side of my design.
Ok, fair enough. I (and I assume others) believe there are all sorts of 'realistic' things that don't actually add to the fun of a game. It would be realistic for sword wounds to leave a lot of adventurers crippled, for example, but I don't find that sort of realism to be a particularly fun way to play RPGs. Some do.
So maybe the question is why does this particular form of realism make the game more fun?
If using completely un-telegraphed, totally random traps, there seem to be a few ways (as I mentioned elsewhere) that this can unfold in play:
1) Players are rewarded for either constantly looking for traps, or randomly doing so and lucking out.
2) Players are rewarded for having a high passive Perception.
3) DM rolls in secret and players are rewarded for having high (normal) Perception.
4) Traps are random consumers of resources by causing damage in unavoidable ways.
Now, a lot of those options are pretty common in D&D, historically. Over the years I've played using all those mechanisms. But, since the "board game" insult has been used by others, those all feel a lot more board-gamey to me. You roll your dice, move your piece, and maybe you land on somebody else's Hotel. Or the lich's death-trap, as the case may be.
So really this comes back to the "player skill" or "challenging the player" thing: I'd just rather play (and DM) where the human players have to pay attention for hints and then use those hints to make meaningful decisions. And by "meaningful decisions" I mean informed decisions with risk:reward tradeoff that will impact the game state either way.