Why can't the su binary simply be copied (techical response please) I have rooted several Samsung devices and the underlying "goal" so to speak seems to be to get the su binary in /system/xbinand install Superuser.apk.

My question is why does one have to jump through all of these hoops to root the phone (install custom recovery and flash pre-rooted ROM or exploit the current installation)?  Couldn't one just download a precompiled su, move it to the SD card, and run it via adb?  The thing that seems to make a ROM "pre-rooted" is that it has Superuser and the su binary in their respective system paths.  I don't see why it is so important that it is ran from /system/xbin.
The su binary needs both the execution and the setuid permission bit set. The first is needed that the file can be executed and the second is that it automatically runs with the file owner's rights (set user id, or setuid. In this case the owner is root. Read more here).

Files on the external storage don't have the executable and setuid permission bits set and it cannot be granted without root rights. Note also that the SD card is mounted with the 'noexec' flag to prevent execution generally to boot:

shell@android:/sdcard $ ./su
/system/bin/sh: ./su: can't execute: Permission denied
126|shell@android:/sdcard $ chmod 4755 su
Unable to chmod su: Operation not permitted
10|shell@android:/sdcard $ mount | grep /mnt/sdcard
/dev/block/mmcblk0p1 /mnt/sdcard vfat [...],noexec,[...]


That's basically why you can't just copy su onto the SD card and then run it to grant yourself root.