Why did they play music over it instead of the actual recording? Were the people making silent films retarded?
The music at the time was live musical accompaniment. There was no music (or any sound) attached to the film itself when these films first screened.
They tried that with some short films (dating back to Dickson Experimental Sound Film in 1894), but it would have been much harder to do with feature-length films at the time. Remember, the recording equipment back then was pretty primitive too.