Why is it considered necessary to nail down the lid of a coffin ?

so when the corpse turns into a zombie,It cant get out.Come on man!! Dont you watch movies?Do you want zombies running around everywhere?Why is it considered necessary to nail down the lid of a coffin ?
To make sure s/he doesn't come back out???Why is it considered necessary to nail down the lid of a coffin ?
In case they drop it???
The Taiwanese do this as part of the ritual...


After the coffin is sealed, the wooden lid is placed on it and a ceremony to nail down the coffin lid is performed. Four nails are driven into the coffin lid in the following order: East, South, West, North. While pounding down the nails ';good words'; wishing riches and prosperity to the mourners are chanted. Then a final nail is pounded into the very tip of the head of the coffin. This is the posterity nail, thought to bless the family with future prosperity.
just in case..... ;)
Making sure that what goes in Never comes out.