Because people are impatients. They don't want wait for the "good oportunity" to bet in the right time. Even if they do it correctly 1, 2, 3 times, at any time it will see some game and it will think "i will bet 80% of my money in this game cuz it is IMPOSSIBLE this team lose this game", in this moment this person start to lose for "the house", because eventually some bet that it will do will be wrong and it will lose more than expected because it was so confident. PS: i'm brazilian, my english writing maybe not be good or coerent