Because librarians have already decided the exact words they will always use to describe a thing, despite the many different ways one might express an idea. Everything in our catalog that is about that thing will have the same string of words attached to it.
A book about how people live in Northern Ireland will always have the subject heading, "Northern Ireland -- Social Conditions" attached to it, so clicking that subject heading takes me straight to the rest of the books on the subject, without having to imagine those 10 other ways to say that. Nifty, eh?
Some Subject Headings used in our catalog:
Northern Ireland has 555 sub-headings and 1329 titles, from Anti Catholicism to Youth Deinstitutionalization.