Broadly because if it has science in the title it isn't one ;)
(except possibly materials science)

You have to draw a line somewhere, otherwise you end up with STEMASSG
(Science, Technology, Engineering, the Arts, Social Sciences and Games)
which would be silly.

 Surely the point is to encourage people to do a range of subjects that
involve vaguely similar skill sets, or at least interest broadly the same
set of people, and are short of good quality recruits.

 I would have said (not having done and a physicist hence prejudiced) that
only they most quantitive end of social sciences looks anything like a
science in skill set required. Most of it, at least in school, is an essay
subject, plus and possibly most importantly I haven't heard many business
leaders decrying the lack of social scientists in the workforce.

Fundamentally at school the cool kids do arts and social sciences, which
is what STEM is at least partly about discouraging.
 <ducks>
   Dave Ansell

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