That's how PhD defenses work, and schools model a lot of other things off that format. For a lot of fields, "quals" are a presentation in this format where you answer questions on your research plan, rather than an exam. Applying to be a professor also involves a "job talk" in this format, and tenure usually does as well.
For why this format exists in the first place -- it actually comes from old church traditions (like most academic curiosities)
https://www.amazon.com/Academic-Charisma-Origins-Research-University/dp/0226109224