Its just because they sit on the left so they think they need to land on the left to hit the middle. The 737 guy said get your areeese on the centre line and it will all be oK and the case is the same for a PA28.
The only reason not to hit the centre line is
1) Grass runway gets worn out if everyone hits the same point.
2) Tailwheel ac land next to the c/l tp keep staight on the runway in absence of all other visual clues.