Placing an engine in that empty space means that there is less space available for the car's crumple zones to deform before the engine intrudes into the passenger safety cell.

A tesla can have its entire front concertina'd down to nothing without anything entering the safety cell. A front-engined normal car cannot do this; typically 50 to 60% of the length of the front of the car is engine. While external parts of the engine may crumple, the cylinder head and main section of the engine by necessity need to be milled from solid metal; These will not crumple, they will be pushed into the safety cell if the collision is energetic enough.