The reasons behind this fact are pretty easy to guess; graduates have spent more time developing not only their specialised skills relating to the subject they studied at university/college, but also their generalised skills like team-work, leadership, research, communication, presentation etc. The more skills you have, the more valuable you are to an employer because you can do more stuff. And the more stuff you can do, the more someone will want to pay you (and the more you can demand more pay from an employer!).