This is self-deception - in fact, in sports betting, the bookmaker's margin is greater than that of the casino in most games. And the longer the distance, the less likely you are to be smarter than the bookmaker. The only advantage of sports betting is arbitrage bets and all sorts of boosters and bonuses (but in fact they only reduce the bookmaker's margin to the average casino margin).
The house edge can in fact be very high when it comes to sport bets, so not only the punter needs to be very skilled in the particular sport they have chosen to make a few bets, they also need to identify which one of the bets has the smallest edge or try to catch some odds which may be out of place due to punters favoring one team too strongly, all of this makes incredibly complicated for anyone to obtain profits long term from the activity, even if there are a few people out there which can do this.
Yes, for example, in tennis at live bets, the bookmaker's advantage is about 8%. In order to beat the bookmaker at a distance in such poor starting conditions, one must either have an insider or see the future. I don't think anyone can do it. The best bets are something like the Champions League final where each bookmaker competes for the audience as much as possible and the bookmaker's margin drops to 4% (and even lower if bonuses are taken into account).