Have you ever seen anyone taking roadkill off the street? I never have. Broad estimate here - I've probably seen anywhere from five thousand to ten thousand animals run over on the street in my lifetime, and never once have I ever seen anyone remove any of them. Yet they always vanish. Also, given my schedule and age, I'm often on the road late at night, so the "it happens when you're sleeping" answer doesn't satisfy me. I know that there are roadkill removal companies and services, but I think they are merely sham companies set up for money laundering purposes. (Anytime something stays in business longer than it should, I declare that it must be around for money laundering. I think there is a lot of that going on.)
After much deliberation, I have determined that the answer to this quandary is that there is a roadkill fairy, which is invisible, and removes roadkill whenever people are not looking. I have also decided that the roadkill fairy's name is Terry, which is good in this situation because it's a gender-neutral name.
Along similar lines, one of the things I've learned to block out living in New York are fire truck sirens. As a kid, if I heard a fire siren, I'd feel a palpable tinge of concern, and hope that everything was okay. But in New York, fire sirens are so common that any sense of worry has been completely numbed. I must see 3-4 fire trucks with their sirens on each week, if not more, yet I pretty much never see or even hear of any fires. I understand that small apartment fires might still necessitate the fire department despite no visible flames or smoke to outsiders, but I don't even hear people say "oh, there was a fire in my building this weekend". And I personally speak with at least 1-4 people in New York a day. That's a lot of people. (Granted they're often the same people. And I have an imaginary friend.) By my rough estimation, in New York, I've seen around 300 fire trucks with their sirens on, have heard of two fires, and seen zero. The disparity is ridiculous.
There may be a fire siren fairy, who turns on sirens for no reason, overriding the controls of the New York Fire Department. I'm sure that is the explanation and there are no other possible explanations, so don’t even bother to suggest your own ideas.