Sometimes it's even done on purpose to get that "old look" on the movies. Electronic Arts last did it in the new Medal of Honor game. Along with the trusty cliking sound of a film projector.
In general the old black and white movies had a lot to suffer from dust and the enviroment. Add this with the huge bright lightbulb necessary to show the movie and you have a lot of distortion.
It's a killer for compression, because how on earth would a compression algorithm know if something is just a spec of dust or the wart of the famous movie actress It gets even more hilarious when the screen resolution or format is changing from one to another during the conversion. You just can't do that without creating some artifacts.