Teachers are in a bit of a weird position. On the one hand, they want to be considered as professionals, as a lawyer or a doctor might be, empowered and trusted to make decisions in the best interests of their "clients" (e.g. parents and children). But they organize themselves as labour, with rigid unions and collective bargaining. Do you know many teachers a year are dismissed for just not being very good at the job? None, yet as a society we have no problem dismissing doctors, airline pilots, whoever, if they can't do the job, despite passing the qualifications.
Given this, all policymakers can do is try to limit the damage that can be done by e.g. the worst 10% of teachers, by imposing strict syllabuses. The other 90% suffer for it - but hey, they are the ones who place protecting their union brothers ahead of the best interests of the students.
Other professions have professional organizations which try to look out for the interests of their members.
The AMA (American Medical Association) is a very powerful lobby in Washington, and has a lot of influence on the profession.
People in IT often belong to organizations like the IEEE, which (though not a fraction as powerful as the AMA) also try to look out for the interests of their members.
In America, most lawyers belong to the American Bar Association, which serves a similar function to the AMA and IEEE.
As for airline pilots, you might have heard of the Virgin Atlantic strike that was in the news lately. It was initiated by the Balpa pilot union.
Many other professions, from electricians, to engineers, to construction workers, to architects, to nurses, firefighters, police, etc have unions and professional organizations which strive to look out for their members interests.
This is a good thing.. as is collective bargaining. They are a valuable check on the ridiculous amount of power employers in the US have over their employees, and a way to attempt to prevent a race to the bottom in terms of employee wages, benefits, job safety, quality, and satisfaction.
To say that teachers belonging to unions somehow makes them "unprofessional" is disingenuous. If anything, it's the other way around.
Furthermore, it's ridiculous to claim that teachers unions want to keep bad teachers at work. They don't. And there are procedures in place for firing them.
Finally, as to "the best interests of the students", what are those exactly? And who gets to decide what they are? The parents? The teachers? School administrators? Congress? The students themselves? Do any of them really know what is in "the best interests of the students"? This is a complicated issue, and the "one size fits all" approach of standardized syllabuses, standardized tests, and bumper sticker rhetoric does very little to address it.
Given this, all policymakers can do is try to limit the damage that can be done by e.g. the worst 10% of teachers, by imposing strict syllabuses. The other 90% suffer for it - but hey, they are the ones who place protecting their union brothers ahead of the best interests of the students.