I think the problem's that we all already knew there was something wrong

yeah i found it to be predictable as well but it was so well made that i still really, really enjoyed it and was even surprised twice. by the choi appearing thing and the lanterns at the end. everything i saw coming basically when it did even and still felt chills.

Not only did we know that something was going to go horribly wrong, but the characters knew as well. I think this would have been more effective had the weirdness built more slowly. Otherwise, the invitees would probably have bailed right away.

Why people watch trailers?
I watch this one, without even know nothing about the plot and I enjoy that a lot.

I don't think it's predictable as some people say. First, what the husband and wife wants from the guest? it's a mystery, we didn't know until the end, you himself said "we already know the husband and wife are gonna do something weird or crazy" but what excitedly? this's the reveal. Second, it's easily can be him the crazy and not them, for example, the scene with the kangaroo, his low temper and the lack of emotion (alleged), can be an indication of violent impulses. Third, It's probably just me and maybe few more, but in some point in the movie I thought it's some kind of intervention, and the fact that it's similar to the title Invitation convinced me that. At last, if there's more than one possible reveal - predictable is not necessary predictable.

BTW, in the poster there's a big clue about the ending.