I wonder why even trade implied vs realized; hedging tail risk? capture risk premium?
Also, volatility skew will play a role.
What if I just short the underlying when its last close is lowest of the previous 20 days (breakout pattern)?
Thought experiment: imagine you're a large institutional investor who
knows, for certain, that many people will do exactly that. What would you do, and what would be the effect on all those hapless bears?
(For every good, highly-liquid stock, there's a whole bunch of professionals watching it for inefficiencies of that sort. Options, on the other hand... say, twenty expirations times a hundred strikes per expiration times two sides, for
each stock... I don't think the algos are
quite that ubiquitous or pervasive yet.)
Also, shorting a hundred shares of SPY for a month would tie up almost $30k of your capital for that long. If I sell an ATM call on that number of shares that far out, my BPR is under $6k. Your notional risk is a bit higher than mine; if the underlying moves the wrong way, my losses happen at about half the rate of yours; if it goes the right way, my return on risk is quite a bit higher than yours. If you can't use options, you're quite limited in what you can do if things go wrong; since I can, I have - well, a number of options.
But I'm a relatively new, inexperienced options trader. Somebody who really knows them could probably add a few dozen (or a few hundred) more reasons.
If I get stopped (market rebound) I lost 1 unit of risk, if it's a winner (market keeps going down) the payout is non linear just like options. I will do the opposite if I am short volatility.
To say what I've written above another way, static strategies like this one can be (and, I'm certain, have been) arbed out. Vol hacking uses probabilities of dynamic future events, and not a sitting duck at which repeated potshots can be taken.