If you can't actually buy an index, why are futures worth so much?

So I understand that futures track the index, and are cash settled. Same with futures option. What I don't understand is why futures contracts are worth so much money when you can't actually buy the index, only the ETFs that track the index. So why are futures worth so much? To me it seems like it's literally just making a bet about the direction of the index, opposed to a tangible commodity.

I trade options on the Qs as a hobby, and at some point would like to move into futures because I like the price action. And I get HOW indexes work, but not the why. Can someone explain? In both simple and technical terms if possible.