Why are dates (of writing or publication) missing in scientific papers?

Scientific papers are usually published in journals and conference proceedings. Publishers report at least the publication year and month, but may also report more information, like the submission date, the revision date and the date of the first online publication.

Your example is just a preprint, not the publisher's version, and does not contain any information. However, you can find the published paper at this link, where you can freely download the pdf with the publication year. You can also read the front matter of the proceedings to find further information.

The particular paper you linked is a conference paper. Its date of publication will be the date of the conference, which in this case was ESORICS 2012, held September 10-12, 2012. One suspects that what you have linked is a preprint - that is the form of the paper submitted to the publisher before it is published. Computer science is often communicated through conference papers, rather than journal papers.

Journal papers almost always do have a date of publication on them, and often a date received. But again, it could be that your are generally seeing the officially unpublished version, rather than the publishers version.