Why scientific articles do not include year/date?
For most purposes, the publication date is more important than the date the work was done. There are some exceptions, but they are few. The author doesn't know the publication date when the work is done. 

The publication date is a kind of meta-data about the publication, not precisely relevant to the work itself (most cases). That meta-data is maintained by the publisher, of course. 

Some work, of course, is time dependent and so the authors will, then, likely say  when data was gathered, and so on.