Because the time between when you've made the decision to place the order and when you actually physically place it is enough time for the market to shift drast

Because the time between when you've made the decision to place the order and when you actually physically place it is enough time for the market to shift drastically. "The number shown" may be different in fewer milliseconds than it takes the signal from your brain to reach your finger, plus the delivery time for the web request to the server. A couple markets have had flash crashes down to pennies. Imagine placing a market sell order right before that happens, selling your bitcoin for $0.15 instead of $15,000. In reality you're usually somewhat safe, but market orders are always a small gamble for this reason.