Why are dates (of writing or publication) missing in scientific papers? When I read a scientific paper I usually would like to know the point in time when it has been written or publicized. I would expect such a date written near to other meta data like the the author's names. 

Unfortunately often these papers lack in such a date. I can't see why. Okay such a paper may be the work of years, but it is publicized at a single point in time. When I wrote my thesis I wrote down the date when I have submitted it to my university for approval. 

Such a date is very useful for finding, classifying and evaluating papers.

An example would be this: 
https://www.cs.ox.ac.uk/files/6487/pwvault.pdf

The papers I read are in the field of computer science. This may be different in other areas.
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.