why teacher education professor positions require at least 3 years of k-12 experiences?

Because experience actually teaching is important. You can't teach someone how to do a job if you've never done it because there's so much more to it than what you can learn in the classroom. That would be like me going to college and learning about open heart surgery but not actually doing it and then trying to tell someone else how to do it--you simply cannot teach effectively if you haven't actually done it. Also, foreign students can and do get teaching certificates.