Post History

As with anything computer science-related that dates back to the 1960s and 70s, things just happened at a whim. Everything was new and highly experimental back then. Nobody knew how to write or design programs the best way, how to organize them or what coding styles that were most readable. The whole field of computer science had just been invented. 

According to wikipedia the name of the term "class" probably originates from the Simula language, which is a very old experimental one from 1960s. 

Notably "class", "object" and "instance" are in themselves very broad and fuzzy words that in a general English context could mean anything. As are "variable", "structure", "subroutine", "function", "member", "property", "encapsulation" and so on. The pattern is clear: these are all originally incredibly vague and nondescript terms.

So basically someone back in the 1960s or so just picks a name for some term and then it sticks. It is not based on some deeper far-sighted rationale regarding how computer programs would be designed in the distant future, 60 years later. Actual object-orientation as we know it didn't really emerge until the 1980s-1990s, and when it did, it used already existing terms.