How much focused attention one will tend to give a prolonged activity that doesn't normally involve their input, feedback, or control (on top of the fact that learning is evolutionarily not an easy thing to do) is certainly relative to the activities one would alternatively have available to them. That is to say, before we had the ability to read about anything, talk to anyone, or play any game at any time or location I imagine it was somewhat easier to not feel the need to distract ourselves. I think it comes down to regulating our dopamine system's objective function to not require frequent consistent reward and allow ourselves the delayed gratification of something like learning from a lecture.

I believe that is one of the reasons learning by doing is highly effective and essentially unavoidable if one is to truly understand something. This applies to purely theoretical disciplines as well, in the form of practicing calculation and thinking about the implications.