Here's why:
* Different Timelines: Classic Sonic is from a different timeline than Modern Sonic. The two versions of Sonic exist in separate continuities within the Sonic the Hedgehog universe.
* Separate Origins: While they share a similar origin story (created by Dr. Eggman/Robotnik), they are distinct characters with separate adventures and personalities.
* Gameplay Differences: Classic Sonic is known for his more rigid, 2D platforming gameplay, while Modern Sonic is characterized by his faster, 3D platforming style.
In essence, Classic Sonic is a representation of Sonic from the earlier games (mostly 2D), while Modern Sonic is the current iteration of the character. They are distinct entities within the Sonic universe, not brothers.