This is because it is typically used to refer to a male person who guards something or someone. However, it can also be used in a more general sense to refer to a person who guards something, regardless of their gender.
Here are some examples:
* Masculine: "The guard checked my ID."
* General: "The guard at the door was very polite."
If you need to be specific about the gender of a guard, you can use the terms "male guard" or "female guard."