Exactly but if you put your money on the group favourites which most of the times will win all the group stage games then that would have been 3 games won already so another 3 along the way to the final. Aside from that, if we think of this promotions as an extra then its a pretty good deal because everyone loves betting outright on World Cup winner
These promotions are decent, but betting on favourites as you mentioned doesn't always work. For example, I never expected Brentford to actually defeat Manchester City who have been consistently winning recently.
Betting outrights on the World Cup winner is even more difficult in comparison. Betting small amounts in these cases is the way to go if you ask me.