Since the career path of a teacher is distinct from most others, the onus seems to be on those who do not want reform: what makes teaching different from so many other jobs that necessitates such a drastically different incentive structure? Put another way, do you think that software engineers at Google should be granted tenure, in exchange for significantly lower pay, and significantly lower accountability?
Obviously, teacher tenure is a gigantic issue, but I'll try to address some of the points here.
First, it bears mentioning that in fact the career of a "teacher" (here, I'll use the term to refer to a K-12 teacher, which I believe is the main meaning of the term in this discussion) actually follows a similar career path to that of a "professor" (college/university teacher). In that context, it makes sense that teachers would have the possibility of tenure just as professors do. (Note that tenure for professors also seems to serve as a replacement for higher pay.)
But perhaps you are against tenure in all contexts? What, you ask, makes teaching different?
- Schools are not a business. The goal of a school is not to make money, it is to educate. This means that teachers' jobs are fundamentally different than industry jobs. While this doesn't obviously directly impact the tenure question, I think it's important to keep in mind that what works for industry doesn't necessarily work for schools since they have fundamentally different goals.
- Schools are integral to communities. Students are more successful when they are part of a sustained community, when they have consistent adult mentors. They're more likely to show up for school and have a positive relationship to it. But "mentoring relationships of short duration may do more harm than good." So schools are more effective when teachers stay in the school for the long term. Tenure is an incentive for teachers not to move schools frequently. There are other ways to incentivize this, for example Google provides many well-known perks to convince us not to leave - great food, a shuttle service for commuters, etc., all free to employees. But schools are unlikely to do that.
- Teachers need intellectual freedom. Teachers are, unfortunately, under the whim of politically run school boards. Without teacher tenure Texas might have lost all science teachers willing to teach about evolution. Just to name one example.
- Evaluation of teachers is not a solved problem. Because of the vast number of variables that interact to create student achievement, evaluating teachers is not a solved problem. It is, for example, drastically harder than evaluating whether someone in sales is performing well.
- Teachers aren't paid well. This last point is large enough to eclipse all the others, in my opinion. Enough so that I'd consider being for the abolition of teacher tenure if this point were fixed (and a reasonable evaluation structure was put into place). Teachers have high stress high responsibility jobs. They're held accountable for all mistakes in the classroom and those mistakes have real consequences. They should be paid like stock traders, if stock traders weren't allowed coffee breaks. For more on pay and on evaluation, see my previous post on Teachers Have It Easy.
For more examples, citations, and clarity (as well as a bit more attitude) see an NYC teacher's take on the issue.