The word "closed" is used instead of "close" as an adjective because "closed" is originally a past participle form of the verb "to close," which evolved to also function as an adjective describing the state of something not being open. In contrast, "open" has always been an adjective in English. So while "open" is a direct adjective, "closed" describes the condition resulting from the action of closing, reflecting a past event or state. This usage is somewhat arbitrary and influenced by historical language development and the tendency in English to use past participles as adjectives for certain states or conditions.

In practical use, "closed" signifies that something was open before but is now not open, indicating a completed action or state change. For example, a store being "closed" means it has ended its open period, while "open" indicates the current state of being accessible or available.

Thus, "closed" is not just the negation of "open" but an adjective that comes from the past participle form of a verb, pointing to the state derived from the action of closing.