The passive voice is preferable when the subject is unknown, unimportant, or imprecise.
The maxim, "Rome wasn't built in a day" is written in the passive voice for two reasons. One reason is that it is more succinct than the active voice (disproving the active voice zealots claim that the av is always more direct); in active voice it is "Romans didn't build Rome in a day." It is also redundant to mention the subject; we already assume that the Romans built Rome. It would only be necessary to use the subject if someone other than the Romans built Rome.
The second reason is that to use the active voice would be to completely change the meaning of the maxim. By leaving out the subject, the maxim implies that any grand undertaking, such as the building of Rome, cannot be done in a short period of time. The subject is irrelevant. To use the active voice would specify that it was the subject (the Romans) that couldn't build Rome in a short period of time; perhaps they weren't smart enough or industrious enough, but someone else could have possibly built Rome in a day. By being general, the passive voice is timeless and universal.
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