That's like saying that investing in the S&P 500 is nonsense, because if it were so easy then everybody would be doing that, and nobody would buy bonds.

The thing is, that there is a risk/reward relationship for everything in investing. And according to this article, the return you are getting on a 3x leveraged ETF are about 20% annually (2x normal stock returns) while the risk/volatility is about 3x. So, there is no free lunch. You are only getting higher rewards for taking on much higher volatility.