The actual answer is aerodynamics. Having a structure in front of the turbine blades disrupts airflow, creating turbulent flow over the blades which causes the

The actual answer is aerodynamics. Having a structure in front of the turbine blades disrupts airflow, creating turbulent flow over the blades which causes the blades to lose efficiency. It's much easier to design the engine to be able to run for a little while after ingesting soft FOD than it is to create a good debris cover that doesn't destroy efficiency.