What happens if the person you send the money to doesn’t deliver? What happens if someone compromises your account and sends all of your money away fraudulently? These are just two basic things that the current financial system deals with thousands of times per day.
And what about fees? BTC, ETH and many other coins are essentially unusable for smaller transactions. Sending .02 ETH (about $45) from my MEW wallet costs .00292 ETH (about $6.75).