I think a part of the soul of the instrument, in the guitar, is the sound that you can get with the fingers depressing the string. You can make a vibrato with your fingers, for instance. You can't do that mechanically. If you had a mechanical system for depressing strings, you would get a mechanical sound as a result.

In an accordion, the soul of the instrument is in the bellows. You can have a mechanical means of making the chord and not get a mechanical sound because the bellows is the part that is used to get the variations in sound that make it sound not mechanical.