Learning on the job is a valid point, but it's often not interdisciplinary. You learn and develop your skills in the areas you work in. Opportunities to expand your skillset and study other topics are difficult to come by in many work environments. Especially if you're in a position that didn't require a degree in the first place.
I agree you don't have to go to a University to learn, but you do have to find willing and qualified teachers and mentors which can be harder than it might seem.