Here's why:
* Figurative meaning: The phrase doesn't literally mean that the opportunity is physically positioned in front of the speaker before they miss it. It means that the opportunity is present and available right now, and if the speaker doesn't act on it quickly, they will lose it.
* Idiom: An idiom is a phrase or expression that has a figurative meaning that is different from the literal meaning of the words. The phrase "before I miss it" is an idiom because its meaning is not literal, but figurative.
Other ways to express the same meaning:
* Seize the day
* Don't miss out
* Act now
* While you still can
Therefore, "This opportunity is before I miss it" is an idiom that emphasizes the urgency and time-sensitive nature of an opportunity.