Because you don't know if they've done the trick. That's why natural language is padded with extra words to add context and help rule out misunderstandings. The

Because you don't know if they've done the trick. That's why natural language is padded with extra words to add context and help rule out misunderstandings. The grammatical rules of agreement are one mechanism to do this. And you can see the phenomenon of over-abbreviation by reading a few debates on twitter and repeatedly seeing, "how can you not understand what I tweeted?!"