Edit: to clarify, I mean when shops have a sign that says “open” versus “closed”. Why is it not “opened/closed” or “open/close”?
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The boring answer for questions like this one is “because that’s the way the people used those words”.
If you go one level deeper and ask “but why did they do that?”, there isn’t always a single good logical reason we can point to.
In this particular case, even though in the modern language the verbs “to open something” and “to close something” seem to be exact opposites, the words have a very different etymology.
The earliest form of “open” in English is as an adjective, the verb was formed from the adjective later. Independently and from a different source English got the verb “to close” and from that it eventually formed two different adjectives: “close” (as in “near”) and “closed” (as in “shut”).
In some similar cases the language did evolve to make the similarly-used words also have similar forms, but in this case it’s possible that the existence of the second adjective with a different meaning is what prevented the change.
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