Psychology samples - why always students?

I look at many psychology papers. It seems to me that most of them carry out their research on 'undergraduate students'. Is this a representative sample - educated, intelligent, young...can we base our psychological understanding of the whole of society on paper after paper that researches this specialised group? Why don't researchers get out in the real world and do study some real people? Unfortunately, when you read a newspaper report about the latest study it usually fails to mention the sample.