Chaosmancer
Legend
There can always be other reasons. And sure you can be more creative about anything in the game. But killing goblins works and is fun. If you think it is better to go another way, instead of making it compulsory or trying to create a moral or artistic ought for it, show people the joy of going beyond killing goblins, kobolds and orcs. The issue is this is a very simple thing that just works int he game and makes it easy to pick up and play. A lot of people like that, and they don't care if posters on a gaming forum find their enjoyment of it sufficiently creative. I think there is also something to be said for why going into the darkness of a dungeon or wilderness and killing orcs resonates with people, and that something has a lot less to do with colonialism and a lot more with myth and legend.
Do you understand how trivially easy it is to make a supervillain and have people understand they should fight them?
"Hey, this guy named The Evil Lich King is planning on killing everyone in the world and raising them as his undead servants. Will you stop him?" Make perfect sense to people who grew up with super hero stories (you know, modern myths) all around them. Being a hero makes sense to people. You don't need to kill orcs or goblins for simply existing when you can trivially just steal supervillain plots and they work just fine.