Because wisdom (and, more to the point, advice and mentoring) is highly context-dependent. It depends not only on your current understanding of the material, which software has trouble measuring anyway, but also on the way you are thinking about the particular problem and the types of related subjects you have worked with. Not to mention your life situation and overall attitude at the time. A good teacher can take all these into account, deciding when to challenge a student versus focus on reinforcement, pulling in examples from related material with which the student might be familiar, etc.

Software-based learning might work for the subset of people and subjects which can be commonly learned from textbooks by self-motivated individuals now: things like programming, math, some science. But many people require the social interaction, and many topics cannot be easily managed in a purely information-dump approach.

Because highly successful people are not necessarily those who can teach (and also, are 'human teachers' anyway). Teaching is inspiration, empathy, enthusiasm, wit, patience. You could be a Nobel Prize laureate and have none of these, and you'd make a perfect snooze-worthy drone (witness some highly decorated profs at some top universities). I'm going to assume you don't have any children because I think otherwise you would see how much a difference person's personality has on the child's ability to learn.

Granted, many teachers suck. In the U.S. we don't have a system that rewards good teaching.

Also granted, the ability of the current technology to bring amazing (human!) teachers to the masses is great and growing. But, teaching is a two-way street, and you can't have much of that with 1 to 1 million ratio.

The relationship between a teacher and student is an interpersonal relationship. It's like a friendship. The effect of a good teacher, as I see it, is not that you get more knowledgeable about a field. You can do that from a book or the internet. It's to motivate you, to make you excited about a field, because they showed you what they thought was exciting about it and you agreed.

Bad teachers can make you hate a particular field for the rest of your life.