Here's why:
* Imperatives are commands or requests. They are not concerned with the time of the action, but rather with getting the listener to perform the action.
* Tense describes the time of an action. Since imperatives don't describe the time of an action, they don't fit into traditional tense categories (past, present, future).
However, we can think of imperatives as having a temporal implication based on the context:
* Present-oriented: "Close the door." (Implies an immediate action)
* Future-oriented: "Call me tomorrow." (Implies an action in the future)
In summary, imperatives are not assigned a tense in the grammatical sense, but they can be understood to have a temporal implication based on the context.