My point is that lines aren't set at "what's supposed to happen." They are set to make the house money. To sucker people.
So I suppose "equal" isn't exactly right, but it's pretty close.
Books are limiting their exposure to loss. Obviously it isn't 50.0000%/50.0000% or they'd never make money, but they're keeping it close to that. They aren't risking $40 in and paying $60 out.
What do you think the average split is?
I remember the money on the Giants/Patriots Super Bowl was around 65/35. And that's the most bet game in the country - a lot of risk there - so you can imagine that the split could be higher on, for instance, a college basketball game in November.
Say you're a sportsbook that's offering a bet on the Super Bowl coin toss, at -105 either way. If you get $1,050,000 on heads, are you going to change the line? No. You're going to take the $25,000 of expected profit and root for tails.
edit: Math was wrong, point remains.