Here's why:
* Their relationship is primarily platonic: In the games and lore, they are portrayed as friends and allies who share a common enemy (Chaos).
* Tikal is a spirit: Tikal's spirit has been sealed for centuries, and while she interacts with Sonic characters, a romantic relationship isn't implied.
* Shadow's love interests: While Shadow has had close connections with Rouge the Bat and Maria Robotnik, there hasn't been any indication of a romantic interest in Tikal.
However, the games and lore leave a lot to interpretation. Some fans might imagine a potential connection between them, but it's not supported by canon.