Why Don’t Planes Fly in a Straight Line on a Map?

The first time I got on the plane, I was going to Istanbul from New York. I was nervous, and I was checking everything about my flight. When I checked the flight route, I saw that the plane was going to make a curve. I started thinking about that. Why was the plane not going straight and saving some fuel? Instead of going through the United Kingdom, we could go through Italy and Portugal.