I think that a lot of this is because AT&T encrypted their bootloader.
That's correct. We have to wait for a developer to make a work around like saferoot since the bootloader is locked. The att variant isn't very popular so we could be waiting a while to get root.
Sim unlock is not the same. That would allow you to use att phobe on tmobile.
GS5 is rooted thru towel root...supposedly.