Well it's a bias of movies more than science fiction writers I think. Of course the humans are the bad guys if it is an invasion proper.
One with humans fighting Aliens in case anyone hasn't seen it - I don't want to share a spoiler, just say that humans don't come out well in this story. It's a satire on fascism.
And that's done from the point of view of the humans most of the way through.
It is quite a challenge to portray a truly alien society. Unless you give them human faces, then it's hard for us to read emotions into their faces. Not such an issue with writing science fiction as portraying it on screen, as you can tell the reader what the characters are thinking / feeling or allow other characters to read their emotions for you. The insect twirled its antenna aggressively :). Or some such. You can't visually show an insect twirling its antenna aggressively. You have to have another character explain that twirling its antenna is a sign of aggression, and then, that makes it clumsy especially if you have to keep doing that until the viewer "gets it".
So that then means either the ETs are very human like - or else limited to cameo roles.
Just to add to the other reasons given here. It's just a suggestion. As to why I can't think of a movie that depicts an invasion by humans seen from an ET's point of view. Might be other reasons, it's just an idea.
I wonder, does anyone have good ideas for science fiction stories that would make good movies of this sort? I thought of a couple, added as comments, but then realized they are not really human invasion stories.
You want one where humans are the very aggressive "aliens" invading a planet, and where the inhabitants of that planet manage to outwit them and win against us much as humans do with alien invasions of Earth stories.
Same story line basically but flip it around, write from the point of view of the ETs, so - it's easier in fiction, as you can just not describe what they look like and assume they are human - then you find out that the horrible invaders wanting to enslave or tear the planet to pieces, or just kill everyone are humans - maybe a bit of the way into the story, and that your protagonists that you've come to sympathize with and root for are the ETs. And then end in a similar way also that the ETs turn the tables against the humans either by accident, or by some clever display of ingenuity just as humans always do in the alien invasion stories.
I think I've read stories like that but can't seem to remember one right now. Can be great fun if well done.