Why do teachers count off for careless mistakes on tests, when sometimes the brain can't help it?

For instance in algebra class, if you add 3 +2 instead of subratcing 3 -2, you will get counted off, but you still know exactly how to do the problem. Or you put the answer down as negative and such and such number when you know it's positive, and know exactly the steps to solve the problem. Or, in chemistry class if you put the answer down in a different form on accident, but you solve it correctly. Like you put the answer down as kcals instead of jules. I know there are ways to prevent careless errors like that, but sometimes you can't control your brain because the brain is powerfull, and there are many ways the mind works. Now if you completely miss an important date on a history test by a full year or 2, than that wouldn't be careless. I just remember how when I took math tests back when I was in high school or in college, some teachers would take off the entire points, and some would take off half, but sometimes I would get a C instead of a B, or B instead of an A, when I knew how to do the test problems, just made simple, careless mistakes. I also liked the teachers back in high school who let us do test corrections to get some of the points back, and we had to explain it in order to get some points back. Just was wonderin now.