Why do passenger jet manufacturers design their planes with stall prevention systems?
Why do car manufacturers install seat belts? Shouldn't licensed drivers be well aware that they should slow down when it's raining or snowing and that they shouldn't run through red lights or stop signs?  

A better analogy:

Why do car manufacturers install anti-lock break systems? Shouldn't drivers know that when their brakes lock up they should release brake pressure and/or pump the brakes quickly to slow the car down?* 

*To be fair, I don't think this is actually taught in driver's education (at least in the US) anymore - my kids learned this from me, but never reported being taught and/or practicing when they took driver's ed. One of the many reasons flying is safer than driving.


Because accidents happen. 
Because pilots are human and make mistakes. 
Because when you're flying in the clouds with no visual references, it's easy to get confused.
Because even with stall warning &amp; prevention systems in place, confused pilots will fight the system. AF 447