Because the in international basketball it is the shape of an old style key hole
A basketball is in a spherical shape.
Originally, the key was narrower than it is today and had the shape of a keyhole, measuring six feet (1.8 m) wide, hence "the key", with the free throw circle as the head
A basketball is a sphere
The key is the rectangle shape underneath the basket that extends to the free throw line. The key is usually painted with the home team's colors. The key includes the free throw line, the 3 second mark, and the paint.
A basketball is generally a sphere.