If an single Orc baby was left on the doorstep of any decent temple, chances are very high they'd take it in and try to raise it as one of their own. But occurrences like that are extremely rare, and don't produce enough Good Orcs to make the slightest difference to the Orcish societal average.
What about Orc prisoners of war, children and non combatants in villages captured by the forces of Good during the (many) defensive wars against the Orcs, Orc outcasts and refugees from conflicts etc.
Any major conflict against Orcs (and there would have been literally dozens at least) would see thousands if not tens of thousands of Orc refugees, non combatants, prisoners of war and so forth captured or fleeing from the conflict.
Do your Goodly nations simply massacre those refugees, non combatants, women and children and POWs? If not (and I certainly hope they dont) I presume (as Good aligned) they treat those thousands of refugees, POWS, non combatants and so forth with mercy, altruism, respect and kindness.
Meaning that even after one sizeable conflict with a neighboring Orc nation, there should be thousands of Orcs refugees, POW's, non combatants and so forth in civilised society. Given enough time, those Orcs reproduce (and at a faster rate than Humans) meaning any sizeable city likely has a fair few Orcs in it, and a lot more Half Orcs.
And what about the Orc settlements? Are your Goodly nations razing them to the ground in an act of absolute Genocide, or are they razing the Churches of Gruumsh to the ground, and trying to set the Orcs on a less warlike path? Surely they actually succeded at least once or twice!