Here's the problem: Volatility is not the same as risk. Risk should be framed as the chance of losing money. An asset can be very volatile, but never go below its initial value. And that's the way stocks work if your holding period is long enough. The risk becomes zero or close to zero once your holding period is about ten years.* Since most people's holding periods are much longer than that, your risk of actually losing money in the stock market is zero. Since your risk is zero, why do you care about managing the volatility? You don't. But if you did, you can tamp down the volatility by simply adding cash to your portfolio without affecting returns very much.
Remember, stocks go up and down, but money you pay in fees is lost forever.
*Cavaet that the future is no worse than the past, but Wealthfront's models are based on the past as well.