Human language in not limited to the here and now. We can talk about yesterday, tomorrow, or worlds that are entirely fictional. There is not a giraffe standing next to me. Negative, conditional, and counterfactual statements are all possible. Knowledge versus Perfomance Linguistics studies what people KNOW when they know a language.
Many external factors can affect performance.
Linguistics abstracts away from such complicating factors to study the true system of knowledge. But we must use what people DO to discover and test our theories about what people KNOW about language. They do NOT tell people how to speak, they study how people DO speak. Physicists do not tell planets how to move. Chemists do not tell atoms how to combine.