Options liquidity is a function of the liquidity of the underlying because in order to make markets, you need to be able to trade the underlying. So when the underlying is heavily traded, the options would be nearly impossible to trade (well, very possible, but with an extortionate spread). Watch the BxA spreads on options on the open (and to a lesser extent, into close)--they'll often be larger than the theoretical value of the contract.
Why not do both at once, far more efficient! I actually haven't looked at the liquidity at night although that's mainly what I trade during the day. Are there MM who will take an order close to mid or is it just dead?
*Mostly* I have found it dead dead dead. Perhaps a quarter of the time I might have actually wished to trade (either SPX or ES), was there actually a market to do so. The other times (as someone noted), the likely taken price would've been beyond the intrinsic contract value, so ", Charley!" I think *almost* every time I have waited (rather than taking a patently shitty trade), I did in fact get a better price after cash open. But too, none of this is something any of us want to make a habit of.....
Not enough volume. At one point, option market was opened during AH as well but it was closed due to not enough volume and not worth the effort.