I can think of a few explanations for this, but one of the big ones is that the licensing requirements for teachers are often designed (deliberately or not...)

I can think of a few explanations for this, but one of the big ones is that the licensing requirements for teachers are often designed (deliberately or not...) as a barrier to late entry to the profession as you describe. There’s been some movement here in the last few years in some places, for example up until very recently if you were a college professor interested in moving to the high school level somewhere like New York - sorry, no, you’re not qualified because you don’t have a degree in education. There are positive spins for the types of credentialling that have evolved around the professions but I’d argue none of them are in the interest of students. Source: teacher and administrator in various countries