1. Inflation eats 2-4% of everyone's purchasing power each year, disproportionately affecting the poor. This inflation is desired by the central planners.
2. The American system of banking regulations introduces systemic risk into the financial sector.
Sure, if you can prove that your transaction was fraudulent, you can get a couple thousand bucks back into your savings account. But you'll have to gamble your money in the market to beat inflation just to break even. And then, every so often, the entire system will collapse and destroy trillions of dollars of wealth.
As an aside, you also get the bonus of having political control of banking relationships, so you can conveniently freeze and take the money of those who find the political winds aren't blowing their way.
Edit: PLEASE don't take this is an argument for Libra (sorry I wasn't clear), merely an argument for sound monetary policy and decentralization.