The "move fast and break things" mentality is not saying that its good to break things, it's saying that breaking things is the price of moving fast. Failure is bad, but so is being over-cautious. We all already know that failure is bad. No one like to fail so we don't need to keep saying that failure is bad. People often lose perspective and think that it's not worth taking a sensible risk, so we say things like "If you're failing, you're doing something right." to encourage them.