#1 - Short: One StuffIt 4.0 self-extracting archive is only up to 12% of the size of the StuffIt Expander 4.0 application itself and one Disk Copy 6.3 self-mounting-image is only up to 18% of the size of Disk Copy 6.1 application itself
You may think that making StuffIt archives self-extracting will just result in "bloated" archives, but why make people who have only dial-up Internet connection waste time downloading StuffIt Expander 4.0 or later, for which just the installer is ~200KB to download, and why make people always need to have StuffIt Expander 4.0 or later, for which the application itself is already ~200KB, when you can make a self-extracting archive with DropStuff 4.0 which is only up to 32KB larger than the archive as if it was not self extracting, encode it with BinHex 5.0, and upload it here?
Why make people who have only dial-up Internet connection waste time downloading Disk Copy 6.1.2 or later, for which http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates... is ~340KB to download, and why make people always need to have Disk Copy 6.1 or later, for which the application itself is already ~700KB, when you can make a self-mounting image with Disk Copy 6.3.3 (but only in Mac OS 8.1 or later using "Create self-mounting image" from http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates...) which is only up to 128KB larger than the disk image as if it was not self-mounting?
(however, Disk Copy 4.2 images don't have to be made self-mounting because http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates... is only 40KB to download, which should take less than a minute on even a dial-up Internet connection, and the application itself is only 24KB, so it should definitely fit on even a single-sided double-density (400KB) floppy disk, self-mounting images, just like Disk Copy 6.1 itself, will only run on Mac OS 7.0.1 or later, and Disk Copy 4.2, which works on System 6, also can't read self-mounting images)
#2 - Additional compatibility
StuffIt Expander 4.5 and later will crash on any Macintosh computer earlier than the Macintosh II and StuffIt Expander 5.x will refuse to install on Mac OS 7.1 or earlier, but self-extracting archives created with even StuffIt Expander 5.5 will still expand properly on System 6 and StuffIt 4.5 self-extracting archives (but not StuffIt 5.x self-extracting archives) will still expand properly even on a Macintosh 512Ke, Macintosh Plus, Macintosh SE, or Macintosh Classic.
Edit: Replaced 'xxx 7' by 'Mac OS 7' - IIGS User