Placing a market order is equivalent to saying "I'll accept $0 for this asset, so long as that was the best price available." Only most people assume it means "I'll pay the number shown, or something close to it" which is only true of relatively stable assets on markets with plenty of liquidity. Making only limit orders available forces the market participant to choose what range of prices they would find acceptable.
Things work in these limited conditions and then people claim victory, like they are doing with the bitcoin futures. We haven't seen the full market force out yet.
Why doesn't an order type exist like: > "I'll pay the number shown, or something close to it"
? Like: acquire/lock all units to fulfill me market trade and if it is less/more than what i expect, cancel.
Most people don't realize a limit order isn't "buy at this price" but rather "buy at the best price in my favor, but don't spend more than this much."
Fill-or-kill means don't put it on the books, just get it now or cancel. You can also typically specify whether it is a partial fill-or-kill too (get it all or none at all).
Of course that doesn't stop low-latency firms exploiting information differentials to get their bids in before your order makes it to the exchange, a process called front-running. It would be illegal for your own brokerage to frontrun you, but pretty much all discount brokerages sell information to third parties, which enables those third parties to front-run. Ever wonder why your Robinhood trades are free?
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.