can you, though?

Reddit had a breakthrough innovation recently in the form of sh!tcoin (MOON token) built on ethereum that allows you to gift people 'moon points' or something by up-voting their comment. Still scratching my head as to why this required blockchain.

You can build smart contracts with dollars through the Ethereum cryptocurrency. If you think you can make smart contracts any other way then you don't really understand what smart contracts are.

I'm not sure what you mean. You can't program the dollar bill. It's a piece of paper.

If you are asking why stable coins pegged to the dollar don't exist, well they already exist.