The United States has endured many assassinations. Even if the attempts are unsuccessful, the deliver a shocking blow to national security and confidence. A personal airplane for the United States is only one part of maintaining the President's physical safety, but it is no less important than any other security measure.
Air Force One plays a crucial role in transporting the President of the United States. The airplane is highly secure, efficient, and can travel to almost any point in the globe without stopping to refuel. Many times, it is the only logistically feasible means of transporting a person of such great importance.
Yes, the President of the United States should have their own personal airplane because it is the safest way to get them across the world. It is simply a matter of practicality. They should not have to ride an airplane with others because the security would be too difficult for everyone.
Same with the Presidential car, the Beast. Which is basically the most supped up car you can imagine, with all kinds of features no regular car has, specifically to thwart attacks and keep the President safe. Again, a lot of the features we’re not allowed to know about. The President of the United States flies on Air Force One. Period.