Teacher Tenure is Wrong

Why are public school  teachers granted life time job security based on length of service rather than performance? Length of service aka tenure does not make you a good teacher for the rest of a teacher's career, some burn out and rob the kids of their education.
It takes an average of 800 days to fire an inept teacher, a drone waiting to retire.  At universities, tenure makes sense as teachers can take time off to broaden their education, and if students don't like a teacher and feel they are not teaching, they can drop the class and choose someone else.  In the public school system, if one gets an inept 3rd grade tenured teacher for example, the kids are stuck with that drone for a whole year.  Tenure makes no sense for public school teachers K-12.  Some school systems have to pay a bad teacher to leave, which is dumb.  The tenure process needs to go out to pasture.  Good teachers have nothing to worry about, and deserve bonus money. Bad teachers should not be protected from inept performance, they should be fired.  At the very least, parents should have a choice in deciding their child's public school teacher, and deserve the best.  Tenure is protecting bad public teachers, hurting the good teachers, Tenure should be eliminated from the public school system.