Why are there even "good" and "evil" gods in the same group anyway? Surely good gods can't demand all evil being destroyed if they are willing to have evil gods as their allies.
Game setup is up to the gods.
The gods put each group of creatures somewhere and gave them starting knowledge and materials. They made goblinoids look significantly different from other sentient races, put them in poorer area and gave them shorter lifespan but increased reproduction rate.
What the people did with it is in their own hands.
The other races started being racist to goblinoids and left them with no option other than to raid them.
But the world has been running for over a thousand years. The real question is, why is this the first time goblinoids conquered a place? Why didn't ever before several groups of goblinoids banded together, overrun an isolated human country, and establish their own? Or how come no orc or goblin ever proved themselves as a good person, been granted a title and allowed goblinoids to settle in better lands?
There were a thousand years for a country of goblinoids to be created. They have weapons they have knowledge. Not all the fault can be attributed to the gods. Much goes to mortals. Mostly to humanoids, for being so racist, that they would never allow goblinoids to prove themselves. And probably, had they ever conquered anything, humanoids attacked them constantly, until they were destroyed.
But I also have to wonder if goblinoids ever tried diplomacy. Perhaps they did, and were refused. Perhaps they did not, and only ever tried war.