how would you do it? brick on the gas pedal? Too much noise? you could NOT push it in without some part of the vehicle sticking out above water. its not a dead fall down past the ramp.
You'd have to brick the gas pedal and tie off the steering wheel to get it all the way in. Pushing would leave it sticking out. A few years ago, someone did this to two cars and aimed them at a local school. One of the cars missed the front windows and crashed into the wall. The other went into the windows, across the cafeteria, and just missed taking out a support pillar that would've sent the 2nd floor balcony crashing down.
My guess is that they couldn't make it work properly or thought they saw someone watching, so they just took off.