Because they don't ALWAYS decay towards zero. They DO rise when volatility in the market rises just like what we saw on Feb. 5 of 2018 and trust me, that won't be the only and last time we see this. MORE are coming. Consider yourself warned. And many traders, during the most recent market correction, had their accounts wiped out clean in ONE day, erasing ALL of the profits that they have made from "doubling money almost every year". So trading in those ETF/ETN's is not as easy as "just short them". They decay over time but when they do rise occasionally, they rise BIG!! The problem is you just never know WHEN they are going to rise.
Because these ETF/ETN's are there for a different purpose, NOT for "buy and hold" investing. They are not as risky provided you know how to manage its risk properly. And yes there are hedge funds that trade them.
I agree. One could get the same results and the same p&l distribution by selling deep out of the money index puts, same difference. Make money every month for 5 years, lose it all in one day. This circus has been around for decades. There are tons of stupid ways to do this. Sell expensive upside calls in grains. Hey, droughts rarely happen. Sell upside calls in oil, we have plenty of that right? Sell upside calls in natural gas, hey spikes are rare right?
All playing on the same theme. Retail trader doesn't know how to trade. Looks for easy money. Loves the positive feedback loop from selling monthly premium against rare events. Blows out his account. Goes back to work at his old sales job selling life insurance. And round and round we go.