By that point do you continue to bet on those favorites to keep winning or does it bore you?
If it was profitable to solely bet on the favorites, it would never be boring, because it would be profitable, and where there is profit, I don't see boredom! 
The point is that safe bets don't exist, and by placing bets on the favorites, sooner or later the gambler will start facing major losses, with little recovery chance, considering the low odds provided by such bets. Let's say that for each bet on the favorite you lose, you need to win another 10 in order to recover the loss. That isn't feasible on long run. And instead of feeling bored, the gambler must feel regretful after all...