Nowhere else to park money.
RE=good but hugely inflated some places
BANK=bail-in risk + zero return
BONDS=total crap, easiest to fudge
OPTIONS/FUTURES=require some knowledge and trading facility
GOLD=inconvenient and technicals point down
EM's=sketchy at best
CURRENCIES=can be good but require knowledge and trading facility
So people will, and do, pay a premium to be in stocks, even though earnings aren't historically great.
There are good and bad aspects to everything, including US equities at current prices.
Unless you are a gold bug Igy, it's all relative.
I still have a lot in cash, about which I'm getting worried. Wells Fargo, JPM, and others are setting aside more for O&G-related losses, but that won't be enough to take them down; what bothers me is derivatives, possible FDIC receivership, and the receiver coming for unsecured creditor depositors in excess of insured FDIC levels (bail-in).
I get less comfortable by the day having cash in certain banks. I have made some RE moves to get money out of the banks, but even there, I wait until good deals come along.