Trance and Techno -- Concentration, Relaxation, and Feeling 'Centered'?

People claim that they love trance (or techno) because it "helps them relax" or it "helps them to be centered" or it "makes people feel good". If that's true, then why don't math teachers play it during lessons? Why don't history teachers and SAT proctors play it during exams? If it's so relaxing, why don't libraries play it over the loudspeakers?