Here's why:
* It names a specific time period. Nouns are words that name people, places, things, or ideas. In this case, 1987 names a specific year.
* It can be the subject of a sentence. For example: "1987 was a significant year for technology."
* It can be the object of a preposition. For example: "The company was founded in 1987."
While it might seem like a number at first glance, in the context of a sentence, 1987 acts as a noun, representing a particular point in time.