Keep in mind that when we fire a teacher for cause, we are destroying quite a few person-years of educator training; if the educator in question is irredeemable, that is a sunk cost. But if the person in question can be made into a qualified, competent, moral educator with less resources than training a new one from scratch, it is more cost-effective (as a society) to reprogram the individual in question rather than put them into the unskilled labor pool.

That reasoning applies to a heck of a lot more than just teachers; if you find it convincing, then you should extend the same line of thought to any skilled field. The fact that we don't should suggest a number of responses. I wouldn't expect school districts, for example, to be eager to fire veteran teachers and replace them with people fresh out of college; they have as much of an incentive to keep talent around as society as a whole does. If a district wants to fire a teacher for cause, it's probably already exhausted most of the second chances and retraining opportunities that exist.

Scarcity issues also play a role in this: removing a teacher from the labor pool is only damaging to education as a whole if it contributes to a shortage of qualified educators. Little damage is done if there are already more educators in the system than it can find positions for. This isn't the case for all types of teacher (I understand math and science teachers are in short supply in the US), but I'd be surprised if every category was scarce, and in any case this is the sort of thing that districts would be aware of and would probably take into account in punitive decisions.