That's not really true.
The bookies' margin on short priced horses is much smaller than on big priced horses. That's to say that if you consistently bet on even money horses you'll win maybe 45-46% of the time maybe 10% less than fair price suggests, whereas if you consistently bet on 33-1 shots you'll probably win than 2% of the time vs a fair price strike rate that should be 3.33%. So generally you will lose less if you consistently bet short priced horses in the long run.
That's why the bookies always like to spin the narrative that favs winning is a result in favour of the punter and against the bookie.. in truth the fav winning is just less bad for the punter and less good for the bookie because their profit on the race will be lower than if an outsider won (remember the bookies' margin on favs is lower... but there is still a margin).
The problem for punters comes because people see short priced horses as "sure things", ie they don't respect the odds. A 1/2 odds on is a strong fav in the world of horse racing but this horse would get turned over 1 time in 3 on average, so hardly a "shock" result it gets beaten. They lump huge sums onto these horses for too little potential payoff.
Professional bettors, or at least serious bettors, always respect and bet in proportion to their bankroll... something which the casual punter has no concept of.