Why do the bars on a raster barcode never pixelate

I have a barcode that is approximately 2" in width. It is a raster image. Illustrator says it is 105 PPI. You can clearly see that the numbers are already pixelated even at this size. The bars look perfect and crisp. No matter how large I make the bar code, the bars never pixelate. The numbers do, but the bars don't. Has anyone ever encountered this?