He talks as if he were the director of the office. Why do we use were instead of was here?

Here is a phrase structure "as if", which means "as if …".

He talks as if he were the boss of the office.

So, what is the actual situation? The actual situation is that "he is not the boss of the office", which is only the virtual imagination of "I", so this is a special sentence pattern called "subjunctive mood". The predicate at this time is "were", even if the subjects are "I" and "you".

Of course, if the predicate is a general verb, then use its past tense.