Because it's comfortable to be sitting behind one's screen where all your maps and details are hidden, and it's much more annoying to get up every time and throw them on the other side of the screen and recover them ?
This is why I specifically said “hiding their rolls” rather than “rolling behind the screen.” I roll behind the screen sometimes because it’s more ergonomical to do so, but I don’t make any effort to hide the dice. The screen is mostly a quick reference tool for me, not a barrier to the players’ sight lines, and if a player wants to crane their neck to see what I rolled, I won’t stop them. Moreover, if you play on VTTs with digital dice rollers, there is often an option to “whisper GM rolls” so only the GM can see them. I always suspect fudging when this feature is turned on.
And because your players trust you and don't suspect you of working against their fun ?
I should clarify that when I say “I suspect fudging” that doesn’t mean “I suspect the DM is working against the players’ fun.” On the contrary, I assume they are fudging
in service to the players’ fun, or at least what they believe to be the players’ fun.
Frankly, ever since 3e, the level of distrust from the players towards their DM has skyrocketed, and can be seen all over the forums. KotDT made lots of fun about this, not without reason as almost everyone around the table is sort of shifty, in particular due to the severely competitive breed of game that they are playing.
I don’t know what KotDT is. But in my experience if players are distrusting DMs it’s usually because they’ve had a bad experience with a DM who broke their trust. Personally, I do trust my DMs, even when I believe they’re fudging rolls. I trust them to do what they think will create the most enjoyable gaming experience for everyone at the table, which for some DMs includes fudging rolls sometimes.
But in general, the players are not playing against the DM, who is just helping them have incredible adventures. Why that level of distrust ?
Again, it’s not distrust. I trust that the DM is helping the players have incredible adventures. But if they hide their dice rolls, I assume that, at least some of the time, they’re fudging the results in service of that goal. Which wouldn’t be my choice, but I trust they’re doing what they think is best.