The real obstacle in education remains student motivation. Especially in an age of informational abundance, getting access to knowledge isn't the bottleneck, mustering the will to master it is. And there, for good or ill, the main carrot of a college education is the certified degree and transcript, and the main stick is social pressure. Most students are seeking credentials that graduate schools and employers will take seriously and an environment in which they're prodded to do the work. But neither of these things is cheaply available online.