Because in the cartoons and what not, they were free to do whatever they wanted with him for the most part, and liking girls is something cool people are supposed to do.
In Japan/lore, outside of the OVA, Sonic's main passion is adventure. That's all he wants to do, that's what he lives for. He owns a biplane just so he can fly around to islands so he can have adventures on them.
And most importantly, Amy Rose and Sonic's relationship is largely based on the joke that unlike many protagonists at the time that go through hell and high water to save their girlfriend/loved ones, Sonic doesn't really care all that much about Amy. She's his self-proclaimed girlfriend, they literally only meet because Amy used her tarot cards to discover that if she went to Little Planet, she'd meet him. Even after saving her from Metal Sonic, he leaves her at Stardust Speedway to go fight Dr. Robotnic/Eggman.
Not to mention, a big note that people often forget is that, Amy Rose originally is only 8 years old, and at this time, Sonic is like 15, so he sees her as an annoying little girl. Depending on your canon, she either uses magic to get older, or the writers just say "hey, fuck it, she's 12 now!"
So basically, Amy was screwed from the very beginning.
Maybe Sonic Boom! Amy has a chance, but that's that.