1) because if the birds strike as a fluid the birds will extrude through (there are strikes where they have extruded through the windows into the cabin ) and won't be deflected
2) because if the birds impact and break the grill, causing it to be ingested the engine damage will be huge
and 3) and the most important reason
the reduced air flow into the engine would have massive repercussions for engine performance across the life of the engine, costing the airlines billions. Any engine manufacturer who didn't do it would have a licence to print money