If there are oracles, why do we need the blockchain?

Block chains are an expensive, cumbersome and technically dangerous way to structure transactions. Things that happen on the block chain happen between people who don't trust each other - so why do we need it? We hire a referee. And that oracle comes by and inspects the windows and decides whether or not they're clean. So though i don't trust them to hold the money, i do trust them not to have the window cleaner say to them, hay, if you declare that the window is clean, i'll give you 50 % of the money. But i think that then you're also getting into this actually very narrow range of trust.