That's because tenure is a tool for research, it exists to ensure a certain level of freedom of research. For every n researchers who use tenure for slacking of

That's because tenure is a tool for research, it exists to ensure a certain level of freedom of research. For every n researchers who use tenure for slacking off or pursuing a crackpot idea (those two are indistinguishable from the outside) there will be one who makes a valuable contribution that would have been impossible without the freedom granted by tenure. Granting tenure has many downsides, but there is also possible upside which is why tenure exist. But that upside is exclusive to research, it simply does not exist in teaching. Don't get me wrong, it's great when tenured positions are held by people who are also good teachers and many are, but it would be wasteful to base selection on teaching. If you want to improve teaching, create better teaching positions outside of tenure.