Dumping all your money into the market ignores that fact that there's a dependancy between how likely you are to need the money and how well the market is doing. Did equities just crash 30%? That means the economy is probably in the toilet, and you might be about to lose your job. I bet that cash portfolio is looking pretty good right about now.
Every dollar should have a purpose and be invested accordingly. If you're investing for a 10-year+ horizon, then you should absolutely put the money in the stock market because you can afford to ride out any drawdowns. But if for example you're saving for a deposit on a house, or if you can't afford to lose the money, then cash is probably the right choice.