It's an idiom, that is, a fixed formula of words. Idioms cannot
be changed without seeming wrong to native speakers.
Consider for example the idiom
kicked the bucket, which means
died. Even though another word for
bucket is
pail, we can't change this to
kicked the pail
without seeming quite ridiculous to anyone who hears us speaking this
way!!! I'm sure you can find examples of this sort of thing in
your own language.
CJ