Why is there such a jump to diagnose which leads to so many misdiagnosing?

Why is there such a jump to diagnose which leads to so many misdiagnosing? Why, also, are so many teachers stepping in and 'diagnosing' children in their class or demanding the parents of children get their child assessed/diagnosed/medicated when the child might just be a typical kid or just younger than their peers (and perhaps just not ready to start kinder yet, for example). I've had quite a few students bring up personal experiences about this very thing and I've always wondered...why?!?! What is going on?