Because some tools might still put a text-layer under the printed so you can select text and copy.
Of course, the artifacts introduced by printing and scanning (especially with contrast turned way up) gives it an air of legitimacy, although these can also be simulated.
Usually I'm in full control of the software myself so I just output X instead of the secret data.
This degrades quality and wastes paper and toner. There are software tools to convert PDF to raster graphics.