CentOS doesn't because RHEL doesn't. If you don't like it, take it up with them. :-) CentOS realizes that many people will want this, so they rebuild postfix and put it in centosplus for your convenience (at least they did for 4 - I'm not sure where they are with 5 on this aspect).
Ok Matt; I've done it.
* download the source RPM. * install it * modify the spec file * rebuild with new spec file.
I has been easy, at least in i386 packages (x86_64 packages rebuild processes has failed in mysqlclient library link).