“Why haven’t plane manufacturers put a grille on the front of the intakes? I guess a person could be then stuck to the grille; but there should be some sort of release mechanism as long as the wheels are still in contact with the ground.”
To release, you would have to stop the engine. Not too smart on take off.
That's why it would have to have a sensor so that it would only release if the wheels were still on the ground. In this case, the intake of a body stopped the plane anyway, and set the engine on fire. I've heard the same about large birds being sucked in that can happen in the air.