Default [Q] WHY can't we just change the GS3 to a different carrier? (US)

I realize this may seem obvious, but everything I've seen is...aside from branding bloatware and changes at that level in the software / firmware, (hereforth "SW/FW") they are all manufactured the same, with same components and specs. Again aside from what's controlled by the carriers, but they are all "built" the same.
Couldn't it ultimately be totally wiped, and have a different "carrier" OS version installed using KIES or ODIN? Kind of like taking a PC formatting the HD and going from Vista to XP or Linux or some other OS. All of the "carrier" SW/FW run off the same internal and mechanical specs, so my thought is they control it thru programmed components, and could / should be interchangeable.

I guess my question is, is there anything that could be hard-coded and locked that would be done at the manufacturing level that would / could be carrier specific, referenced by the SW/FW that could prevent it.

Considering it's even possible to change the ESN / IMEI numbers now thru a hack, what prevents this?

I know we've all been beaten into believing this can't happen because in the past it couldn't, different cameras or processors or LCDs were used by carrier request, as carrier "features", But now it seems, at least on the SGS3s everything is done at the SW/FW level. So if the components exist the same way across the board (as the seem to w US versions of the GS3) wouldn't ANY SW/FW be interchangeable, because all the requirements would be the same?

And the just like the old CDMA / GSM "can't happen", and now we do have many phones that have the same radio's and are capable of both and it just has to be "unlocked".

I'm using a Verizon Motorola Droid Bionic on Straight Talk with an AT&T Sim.
To do that, I've Rooted, changed the build.prop (system file), and done a NV / Seem edit using RadioComm. I've done other hacks and changes, but my point is, there are things we can do now, that weren't possible before because manufacturing has found it easier and cheaper to use the same components and carrier lock or change them thru SW/FW programming.

Please don't answer if you don't know EXACTLY why.

Also please before answering see this thread...it's kind of a "partially" done way, but the INTNAT'L versions do have different manufactured builds.
http://forum.xda-developers.com/show....php?t=2406482