The games sell well enough for them to keep making them.
Give 'em time. People are going to start noticing every new Sonic game is s**t and they'll stop buying them.
List of other things SEGA f**ked up:
Sonic 06
Sonic and the Black Knight
Shadow the Hedgehog
Sonic Forces
Sonic Unleashed
Sonic Boom: Rise of Lyric
Sonic Boom: Shattered Crystal
Sonic Blast
Sonic the Hedgehog Genesis (GBA)
Sonic 3D Blast
Sonic Lost World
Sonic R
Sonic Riders
Sonic Free Riders
Sonic Labyrith
Sonic Shuffle