Thank you for all your replies, everyone. I appreciate them very much.
It's just part of the metaphor.
The borrowed meaning for “window of opportunity” might have originated from an actual window.
Other example of a borrowed meaning, which has not specifically to do with the issue, but offers a broad idea of borrowed meanings.
Yes, this type of expression is very interesting. (Thank you for your detailed explanation, again. It helps me a lot.)
Until I got your kind explanations on this thread, I didn't know "window to opportunity" is translated as 機会の窓. So far, people who use 機会の窓 is still limited, though. (e.g. Medical expert, diplomat, financer.)
We have a very similar(?) phrase, 社会の窓. The literal meaning is probably "window of society" or "window for society". You might think this is also abstract. We use this phrase with verb that is used for an actual windows. While "window of opportunity" are used as an abstraction, 社会の窓 is not.
If someone say "社会の窓が開いてますよ" to you, you might blush.
"window of opportunity" is much understandable than 社会の窓．
社会の窓 is one of oddest metaphors to me.