Ordinarily, parlays rake in profits for the sportsbooks. In fact, through most of 2021 the state of New Jersey had made $608 million off of its bettors. $335 million, or over half of that profit, had come from parlays.
This is because when a parlay is placed, the edge (or lack thereof) is compounded for each leg of the parlay. So on a standard -110 bet, the sportsbooks have that ten cents of edge.
If a five-leg parlay is placed then that edge the sportsbooks have compounds five times, putting the odds in favor of the books.
The way to beat this and make profitable parlays is to make all legs of the parlay positive expected value, or +EV. If all legs are +EV, then your edge will start compounding instead.