It doesn't work just fine at all. I could tell you plenty of stories and industry insights but I'll leave you just with one nugget I got at a market-maker conference last year: not even 20 cryptoexchanges are profitable, and this information comes from those who are paid to make markets.. move.

edit: I think maybe you were referring to ForEx currencies only.. well, there's a difference between a broker and an exchange and I'm constantly surprised traders don't know this.

PS: I worked at Euronext too (many years ago)

That's mostly because the vast majority of people holding currency aren't doing so to speculate on its future value.