The Answer is:
If a vehicle drives in a straight line at high speed for a long time, it will cause the driver's vision to become fatigued, attention to be distracted, and even lead to drowsiness, which is unsafe. Therefore, driving on a road that changes from straight to curved will stimulate the driver's concentration better.

Not to mention, builders also consider that a powerful vehicle can easily climb a steep road, but a heavily loaded truck or a passenger vehicle cannot. Moreover, a road with a straight, steep incline is extremely dangerous when descending, as it is difficult to control speed. A steep incline can also affect the driver's visibility, making it hard to see ahead.

When designing curves, builders try to increase the curve radius and reduce centrifugal force, allowing vehicles to handle the turn more easily even at high speeds.

In summary, while winding roads may take more time to travel, they ensure greater safety. Driving on a steep, straight road is akin to playing a dangerous game that most people wouldn't want to try.