I would say because there are things that you can more effectively directly from others. It's easy to learn facts by yourself. It's more difficult to learn what they mean, or how to analyze them, or (sometimes) even how to pronounce the terms. (Side note: maybe this is why there are such great debates in the tech world about pronunciations...)

That's not to say you can't learn remotely, but rather than in many fields, it certainly helps to have a teacher/guide.