Because they believe what they are teaching is hard stuff. Especially in math. And they've heard that if you skip over anything, the child will not have a good understanding of basic concepts and will fail algebra. The teachers have this drilled into them that kids get math phobia, that kids can appear to understand, but not really understand. This makes them worried about advancing kids unless they are really really sure. And they hear stories about teachers that got it wrong and the kid struggled later. And they don't want to be that teacher. There is nothing that they can confidently look at to say this child has mastered this because there is always a reason in the back of their head why they may not be seeing things clearly.
One thing I'd add to spaghetti's insight above.... we found that most of the teachers we dealt with in elementary school (at my ds' first school) weren't people who had a core strength in math themselves; they were often people who'd found math challenging when they were students, and they didn't really understand that math is easy for some people.
polarbear
