In the given example if a buyer wanted to buy at $20 instead of $20.10 they could easily put in a limit order for that price and wait for a counter party. That

In the given example if a buyer wanted to buy at $20 instead of $20.10 they could easily put in a limit order for that price and wait for a counter party. That would be taking a risk though that either: A) a counter-party never showed up B) a counter-party only showed up when the stock dropped so you end up buying on the way down. Taking this risk is a perfectly fine thing to do and many people do this. Others would prefer to pay the market maker a small fee for providing liquidity. That fee is not being sucked out by some HFT. It's a fee for providing a service. And BTW, thanks to automation and sophisticated trading algorithms that service is provided at EXTREMELY low cost these days. Bid/ask spreads are tiny.