Because the internal_SD is on the motherboard.
Change the motherboard and you are effectively changing out the internal_SD (plus CPU, USB, headphone jack,...). The people you read about may have an internal_SD but they won't have all their old stuff. It'll still be locked up on the old internal_SD that was attached to the old motherboard that was swapped out.
New motherboards are about $40+usd on Ebay. Or you could buy a damaged phone and depending on where the damage is (ie glass or digtizer), you may luck out and get a full functioning motherboard with working internal_SD.
"The work of men of genius, however wrongly directed, hardly ever fails to open the eyes of men to new possibilites."
- Mary Shelley