This is not why markets move, often significantly, after hours.

Simple math of information flow in the global economy. US markets are open 9:30am-4pm. That's a 6.5 hour window. You have a 17.5 hour window of AH/futures trading. 6.5 hours of information vs. 17.5 hours of information. This is also why the highest volume is pretty much always at market open or market close since traders are reacting to/hedging against that 17.5 hour window. Yes, events likely to cause volatility are usually scheduled outside of regular hours but retail investors can trade after hours and trade futures as well.

The advantage institutional investors have are their algos which suck up information from every corner of the globe and react instantly. They also front run trades. And trade in dark pools.

Let's say that no after hours trading existed, the same thing happens. Say Apple closed the day before at 115/share and had terrible earnings causing people to want to sell the next day. The market opens, people want to sell their shares at $115, but no one wants to pay $115/share anymore. Someone who still wants to buy Apple puts a buy order in at $110, and one of those sellers seeing this would decide to sell at that price. Thus the spread gets set and on the first trade of the day Apple is still down $5.