Because there has to be a standard.
As you noted, there are schools who will take in anyone -- teaching degree or not. You even mentioned someone being hired who was a murderer. I don't know about you, but that does not sound like a school I'd want my kid to be in.
A teaching license, along with things like background checks and similar, ensures that there is a standard. Education is a practice, and you have to follow the same rules for that practice like everyone else.
It's the same as engineering and astronomy. Just because you know your engineering or your astronomy well doesn't mean you can automatically be hired on as an engineer or an astronaut. You have to go through the practice of getting certified to do these things.For engineering, you have to pass tests like the IEEE exam. For being an astronaut, you have to have a Bachelor's in engineering, biological science, or a few other STEM majors, along with passing NASA's physical exam.
The same is applied with wanting to be an teacher. Moreover, it's not just about teaching the subject. When you are a teacher, for the few hours you have those students, you are responsible for them (especially in high school and grade school). The Board just can't hire anyone who knows their stuff because they still have to be trustworthy enough to work with minors; again with the murderer example -- why would I, as a parent, trust a school who would hire anyone just because they might be good at math or science? Them knowing the subject does not negate their actions, and it wouldn't make me feel safer about my kid being around them.
Of course, there are people who pass the standard and are still not eligible to be a teacher. At the same time, we still have systems in place to get them away from the students. My high school had a sports coach who turned out to be a predator and preyed on one of the freshman girls. The school had checked his records beforehand and there was never a case of sexual deviancy. They still fired him, still sued him and supported that student. He was skilled in sports and knew his stuff, but that did not negate the fact that he preyed on a student.
The system and practice is there to make sure we have people who can be trusted with students. And if they can't be, we make sure they're not around them anymore. This is more manageable than allowing anyone off the street to come on board just because they know a subject. Because when we do this, we are basically saying that we don't care about the safety or health of our students. We forgo responsibility towards them.
Sure, they know their stuff, but are they going to be a good teacher? Will they actually help develop a student's growth and not stunt it.
I also ask this -- would you say that we shouldn't have standards on who we allow mandate our FDA policies? Our Drug Regulations? Would you say the same for law enforcement or emergency specialists? Just like teachers, they still need certifications for their practices. They still need to go through the proceedings to make sure they are trustworthy.