Trust and cost. Referrals even if the company pays $1-5k to the referrer are fair less expensive then paying 15-30% of the first years salary to an agency. And the trust: if the referral is coming from an inside person that has knowledge of the systems etc, they are referring someone that they feel will work well in that environment and that will fit in with the team. Team fit is almost more important than having all the skills day 1 most of the time.
For new grads, this means using your friends and connections (professors etc) from school to help land a job. In the absence of that ability, get out and meet people and show employers you are awesome. For developers, creating a public git repository is an awesome way to let them see your skills.