Because there was never said that he actually originated from Outworld.
And who said that he was human in the first place?

If you read my post with the link to Shang Tsung's true form, you can see the guy isn't human. Or at least wasn't intended to be in the first place.
The point I'm making, since you're not getting it is, If Bo Rai Cho would earn victories for outworld, even though he fought on the side of Earth, Shang Tsung's victories should've been added to Earth even though he was fighting for outworld.

i only theorised about sub zero and reptile now shang is the mix its abit of awkward dilemmna. ok we'll try and break it down.
its official according to deception that bo rai cho cant compete in an official mk tournament by official i mean one held evry 100 years, well he can but if he does his victory if he won, would be a victory for outworld hence why he only trained liu kang to fight. saying that how many of the mk games have been an official mk tournament 1 to my knowledge the first all the others where blags and cons.
its a rumour inspired by the mk development team that sub zero may have lineage from outworld. One has too assume he was born on earth. the question there is if im an englishman born in china am i chinese or english? i believe the answer would be chinese. in that case i dont think it matters who his family are and where there from but who sub is and more over where he's from, hes from earth so i think his victory is a victory for earthrealm. i ammagine the same would apply to reptile and outworld.
As for Shang Tsung i cant really theorise anything cuz ive not delved into his past in the slightest really. Its got to be a victory for outworld cuz he's a demon sorceror who was bannished to earthrealm. oh i dont know about this shang thing.
and to be fair i only said that stuff about sub turning bad and scorpion turning good cause i think scorpion is awesome and sub-zero is the cheezy token character every new mk player goes for like ryu in streetfighter that and i never tire of throwing harpoons in the blue guys throat. now iceman from x-men there's a freezing guy you can be proud to pick...lol

Ont eh Shang thing, think about it like this...
Shang was born on Earthrealm but wanted more power and so he went to Kahn for it (or maybe Kahn needed/wanted an inside man and so he contacted Tsung). Any way, in order for Shang's victories to go to Outworld Kahn brought him to there where he was reborn as the demon sorceror he is now.
Not official but a good expanation.
About Sub...I've got nothin, except that that him and his bro are most likely twins. I mean he's pretty cut and dry as a character if you ask me, not many holes at all.
His father was a lin Kuei warrior who took a wife, or just had a fling or two. And when the male children of that relationship came of age he forcefully took them from their mother and introduced them to the Lin Kuei. The older twin, was seen as the stronger of the two and eventually, one of, if not the, strongest member of the Lin Kuei. When he was killed by Scorpion, the name was past to his younger bro and the adventures of the new sub-zero began.
All the Cryomacer stuff came later after Sub's origins had all ready been established.