Well, to the first point I'd say that's on the DM. Not to say it's a bad thing, just that it was the way it was because of a choice they made in their portrayal of goblins.
As for the second, let's step back a bit and look at things from a wider scale. What makes a "humanoid" a "humanoid"? What makes something "evil"? Not just from a D&D sense, but let's look at fiction as a whole. "Humanoid" is an arbitrary classification for human-like. Goblins are "humanoid" because they're more intelligent than animals and have a roughly human shape. That doesn't mean they are the same as humans in every way but physical. What defines a "humanoid" says nothing of what the eat or if they're predisposed to aggression or anything of the sort. What about "evil"? This is a big problem I have with D&D specifically in that good and evil are objective things. A dragon burns down a farmer's cottage and when the family flees, snatches up their son and flies off with him to devour. Is the dragon evil? From a human perspective, yes. From the dragon's perspective, maybe it was just hungry. In either case, if you're an adventurer passing through and willing to help or even just a foolhardy member of that village who doesn't want to it to happen again, the dragon is a threat and an antagonist to be dealt with. My point is that goblins, dragons and the like are fictional monsters. They are only as "human" as the person telling the story makes them. Maybe they're intelligent and able to converse with humans and understand morality or maybe they're more primal and see humans as little more than dinner. Or they could be something entirely more alien. At the end of the day, they're fictional monsters and the rules are what the author makes them.
And as to why does it always have to be goblins? It doesn't. If you want to tell that story, then go ahead and tell it. People tend to like what is familiar, however, which is why the cliche evil army of goblins/orcs/whatever is a cliche in the first place. It's why most fantasy settings that have species other than human usually go with dwarf and elf first.
Well, in the campaign I'm currently in, goblins are a caste of a species of semi-intelligent asexual bipedal fungus who enjoy violence above all else to the point that they'll gladly pick fights with each other if no one else is available and reproduce by dying. Warhammer is funny like that.