They have to compete with others. If the 'good' betters go with you that means you have worse odds that your competition and anyone else who knows goes elsewhere.

the real money is in pools where only a few people bet. however that works best if you get people to bet on the unlikely thing you want to take the other side as you can win more often. Often the unlikely thing is only possible because you take a side - so the savey better is taking away your bets.