[jewel]

For me at least, it's because those without a few years of experience aren't going to be able to be productive enough to be worth their pay. In fact, even if money isn't an issue, they can add more workload to the senior members of the team in mentoring and bug-fixing time. This might be partially because web development changes so fast. It's unlikely that someone with a degree but no real-world experience is going to be productive without (optimistically) a few months of training.

Don't get me wrong, I'd gladly hire someone who has done her own side-projects that show initiative and ability to teach herself, even if she wasn't through school yet.

[VOYD]

Plus there is some experience that you just can't get until you are part of bigger (i.e. "enterprise level") projects, and you've learned a lot of the "gotchas" that come along with that.

[lsiebert]

That's why, in part I'm spending some of my time in between job hunting working on contributing to an OS project, not because I'm going to be learning the latest django or javascript idiom so I can hit the ground running, or detailed algorithms for solving a complex problem, but because working with teams is a soft skill that takes time to develop. Also I use the project everyday.