I always found it odd that Mac OS X's "Connect to server" menu let's you read and write to every protocol it supports but FTP. Is it just one of those mysteries, or have they ever discussed why this design choice was made? Your conjecture is welcome too.
Apple rarely discusses these kinds of decisions, so don't hold your breath for any "official" answer. Many requests for this feature have flowed into Apple's bug tracking system over the years, but so far it seems that the demand is not high enough for Apple to dedicate any resources to implementing it.
In the meantime, there are many excellent full-featured FTP applications for the Mac:
And don't forget the
@siracusa is right, we'll never know the reason.
If you want the file system style access that the Finder offers, but with write support, there is the MacFUSE project. There are two GUI programs for managing these connections MacFusion (donationware) and ExpandDrive (commercial w/trial).