<tongue in cheek>
As a GP – you know when patients come in to see you and say “I’ve googled it – I have XXXX problem and I think the solution is YYY” ?
Speaking an engineer – just like that.
</tongue in cheek>
For OEMs it’s much easier to bang in a Star-Fangled-Nut than tap threads.
For consumers, it means you can put the SFN further down if you want to if you cut your stem down, )rather than have the whole stem threaded)
Finally – internal threads would significantly weaken the structural integrity of the stem – it both removes material and created stress raisers at the “bottom” of the threads, so for the same factor of safety stems would be thicker therefore heavier. Realistically, most stress is on the bottom of the stem where it joins the steerer, so this is probably irrelevant anyway.
I seem to have undermined my original point, but never mind.