It's not an entirely unfounded bias, either. There're a lot of books online. Some of them are bound to be crap. But, as a couple of people pointed out in the comments last week, there's crap coming out of traditional publishing, too. Even some books lauded as "bestsellers," that make millions, have their detractors.
Self-publishing's been stigmatized because it seems to allow anyone to vomit something up there and drag us all down with 'em. There's no oversight, no gatekeeping. It's interesting, because complaints of unfair or erratic gatekeeping are perhaps the most common of those leveled against traditional publishing. (eyebrow: raised)
Self-publishing is coming into its own as a viable option for serious writers, and traditional publishing is actually leading that charge, primarily from the agenting side. Agents are a great legitimizing force for self-published authors because, in general, recognized experts raise the perceived value of a product. Raising the perceived value = eliminating stigma.
Self- and traditional publishing are not foes, and they're not either/or. That's why traditional publishing must-haves, like agents, aren't going anywhere. It's why, despite all predictions 5 years ago, publishing hasn't gone bankrupt and stopped printing physical books. It's ironic, but the most important people in self-publishing's journey to validity are traditional publishing professionals.