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.
In addition to the engine going under the vehicle (even at fairly low speed crashes; just watch NHSTA videos on YouTube), the front motor in a Model S is more dense and weighs about the same as a 4 cylinder engine, transmission and accessories. I'm gonna call that a wash until I see evidence to the contrary.
Straight on collisions aren't even the toughest ones to pass. It's the small offset tests that really seem to give manufacturers a hard time. And no engine there.
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.