As others have said, volume isn’t the point. A 12-string guitar, whether acoustic or electric, is meant to achieve the chorus effect.
Think about what a chorus pedal does. It delays the sound and mixes it with the original signal. It also detunes the signal and mixes it with the original signal. If it’s stereo chorus, it will mix up the detuned and delayed signals in the stereo sound-space.
Now think about a choir. Why does it sound different from a single person singing? It isn’t about the volume. After all, a choir of 10 people isn’t ten times louder than one person. It’s the fact that the singers, no matter how good they are, aren’t hitting the exact same pitch, and they aren’t doing it at the exact same time. What people call the “fullness” of a choir isn’t just the harmony; it’s also the chorus effect.
Some instruments can achieve the effect. By doubling the courses, a 12-string guitar does it quite effectively. The strings aren’t hit simultaneously; they’re strummed. And 12-strings, no matter how well tuned they are, never hit the exact same pitch.
Pianos have multiple courses for most notes, so piano does it as well. It’s just so intrinsic to the sound of piano that you may not notice. Synthesizers are capable of it. By slightly detuning multiple oscillators, the effect is achieved.