Because they don't have a better alternative. This is a better alternative. Invite them to technical seminars. Invite them to open houses or beer busts. Pay them to work on mini-projects.
Offer feedback about their Web pages.
It's the responsibility of everyone at the company. If you're serious about tapping great talent, you have to devise ways to identify talented people before other companies do. Over time, you'll develop a talent database. Don't just check the references that talented applicants provide. Consider people who offer references as job candidates in their own right, and capture their names as well.