Why are U.S. Indexes so coupled?

[pianodoctor]

I've had a few general investing questions in the back of my mind lately that could be put up for general discussion- wasn't sure how to categorize them for iTulip purposes- so finally I figured I'll just make them "Ask iTulip".

Q1: I have observed such an incredibly tight correlation between the Dow and the S&P500- whether on an intraday minute-by-minute basis, or short, intermediate, or long term. It's incredible how they track each other. Like, if you wanted to do that on purpose, you'd think it would be impossible- but there it is. It's almost as if no one is doing anything other than trading index funds. Is that what's going on? If not, what?

I'll make separate threads for my other questions.