Because of the speed and force that large poo's vacate the rectum, they carry great momentum and such turds are difficult to stop, therefore, they keep going all the way through the toilet so that you can't see them upon peering down the loo.
Such dumps are very frustrating to a shitter as they leave no sense of achievement, as big craps require hard work.
John: Dammit! I just did a ghoster.
Tim: Ohnoes! I hate it when that happens!
...some seconds later
Tim: Why were you looking down the toilet?