Content Knowledge vs Pedagogical Skill
Ben Orlin (the author of Math with Bad Drawings) wrote fairly eloquently on this a few years ago. His thesis is that instructors are people who (broadly speaking) needs expertise in two somewhat unrelated areas:
Content knowledge: an instructor must have mastery of the field that they are teaching. An instructor should know the details of the topic that they are teaching, and should be able to reconstruct or recall those details immediately.
Pedagogical/andragogical skill: an instructor must have an ability to effectively communicate ideas and maintain engagement. An instructor should know how to keep students interested in what they are teaching, and what kinds of experiences are likely to lead to generation and retention of knowledge.
Orlin further argues that the traditional viewpoint of education is that instructors of young students needn't have a great deal of content knowledge, but must be masters of pedagogy; while instructors of older students must have a great deal of content knowledge, but needn't be great pedagogs.
While Orlin tries to dismantle this traditional point of view some, I think that it is an accurate description of the current status quo.
With that in mind, licensure requirements for primary and secondary education (but not tertiary) are somewhat explained by what the licensure ensures: (1) that the instructor knows how to work with kids, (2) that they are not dangerous to children, and (3) that they have the minimal content knowledge required to get the job done.
Of these, (1) is probably the most critical. As an undergraduate, I underwent a teacher certification (to teach middle school and high school mathematics). Nearly all of the courses I took for that certification were related to pedagogy. Thus my license certifies that I have the pedagogical knowledge necessary to teach a class.
At higher levels, where content knowledge is more highly prized, an MA or PhD is generally sufficient to act in lieu of a license.
Children vs Adults
Another compelling argument for licensure is that students in a primary or secondary education setting are not adults, and need to be protected (by the state). This is doubly important in a society where student are required by the state to enroll in school (up to a certain age or level of education).
If a government required that students enroll in school, but then did nothing to ensure that the education those students are receiving is adequate, then one can imagine that this requirement would quickly evaporate as a kind of "unfunded mandate".
Children have very little choice about education. Most children (in places where licensure is required; or, at least, in the US and western Europe) attend publicly funded schools, where their teachers are employed by the state (i.e. by a municipal government, by a state government, by a provincial government, etc). Because the state is responsible for educating students, and because the students don't have much of a say in the matter, the state takes on the responsibility of ensuring that the instructors can do the job. This is typically done through licensure.
In contrast, adults are generally not required to attend post-secondary institutions, and if they choose to attend, they generally have more choice regarding which institution they attend (even a student with poor grades in the US typically has a number of community colleges and state colleges to choose from).
This lack of a requirement combined with the choice of institution puts significantly less pressure on the state to ensure that instructors are adequate (and, as above, it is assumed that they needn't be adequate, anyway), hence licensure is far less common.
The Role of the Academy
In the American system (and, I presume, elsewhere, as well), primary and secondary education are meant to ensure that the citizenry has a basic foundation of knowledge and skill; and primary and secondary institutions are meant to provide instruction, and have no other role in society. You attend such an institution in order to be taught, and the people teaching you have no job other than providing instruction.
By contrast, university, college, and (even) community college instructors typically have a much broader job description. The primary job of a member of the faculty at a university is generally "conduct research and produce papers". These folk are not (usually) hired to teach classes, but do so as part of training others to take their place—higher education is an apprenticeship program for researchers.
Because licensure is largely about ensuring some instructional quality (rather than content area knowledge), there is little call for university faculty to be licensed. Again, an advanced degree is usually enough to ensure that a minimal threshold is hit.