Component's and GameObject's are inherited types of a base class called Object. Component's can only exist and be attached to Objects.
A Transform is an inherited type of Component. All GameObject's must have a Transform Component attached to them. The reason being:
Every object in a scene has a Transform. It's used to store and
manipulate the position, rotation and scale of the object. Every
Transform can have a parent, which allows you to apply position,
rotation and scale hierarchically. This is the hierarchy seen in the
Hierarchy pane.
If you asking why such an architecture is in place, you may find that this sort of discussion will be found debated in more than a few articles and books. I once asked this question myself and I found the following Stack Overflow answer to best answer it HERE.