Why can it be dangerous to overtake on an unobscured, straight road?

Your answer is fully correct. On a straight and clear road, overtaking might seem safe, but it can still be dangerous because drivers often misjudge important factors. Many mistakenly think oncoming cars are farther away than they really are, which leads to underestimating risk. It's also common to misjudge the speed of an approaching vehicle, making you think you have more time to complete overtaking than you actually do. Lastly, drivers tend to believe the distance needed to safely overtake is shorter than it actually is, especially at higher speeds. All these misjudgments increase the danger, even if visibility seems clear. That's why it's important to always double-check your estimations and remember that overtaking involves more risk than it may appear.