"It seems ridiculous to me that I, with a couple of degrees in history but no public school teaching experience, would have an easier time getting certified to teach that subject at the secondary school level than a person who has been teaching high school history for his entire lifetime but whose degree is in public education!"

In all honesty, why? You're "seems ridiculous" assertion assumes:

1) That the teacher who has been "Teaching...history for his entire lifetime" has been doing so successfully.

2) Your knowledge and experience as a lifetime learner would stand you in poor stead as a teacher.