Because it has a method known as draw. Which considers the text attribute of the sprite so we can store surfaces , images and rectangles .But what if we want to store the circle sprite which requires position and radius to be drawn
A sprite is an image; images happen to have rectangular shape because that's a natural consequence of the way they are represented.
If you want to draw a circle, you have two options: you can draw an image of the circle, which would be a sprite that you position like any other sprite, using its corner as its reference point; or you can draw an actual circle, where a sprite library would not be a huge help.
Can only be rectangular objects stored in pygame.sprite.Group