The simple reason is that the impact force increases as the square of speed. Start doing the math of even a 2.5 lb bird hitting a surface at 250 knots and the force is high enough that building any sort of effective screen acnnot be practically done.
Take a look at bird strike damage pictures at even low speed and you will see that even airframe structures are not immune to damage.
This is a great theoretical idea that surfaces regularly, but is currently an engineering impossibility.