Because on the contrary, the chances of the casino's favorite to win is always 60% possible, which is why a newbie gambler who has absolutely no knowledge about a sport game could predict and win, unlike in certain rare occasions that the opposite might win.
Assuming I agree with you on that win percentage - what if the favorites you're betting on only have average odds of 1.50?
Even if you’re winning 60% of the time, you’d still lose in the long run. A gambler will eventually notice that, since the bankroll will keep shrinking despite having more wins than losses.