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Because they're more interested in maximizing their expected value than in minimizing their variance, of course. Bankroll is no issue for a casino (or other large sportsbook), so they can afford to take the variance hit if they're convinced that they can improve their EV that way.

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These are big corporations with stock on the NYSE and boards of directors. They want guaranteed (ie juice) profit and will do everything in their power to minimize the risk aspect. The risk aspect being exposure to one side or the other.

This doesn't mean that they always get 50/50 action which would be ideal but that 50/50 action is what they strive for. When the difference between a 3 point and a 3.5 point line will shift the balance one way or the other I have little doubt that they use that power to shift it the way they want to be on the side they think has the best chance. But if they could shift it to 50/50 dead ass equal on every game that is exactly what they would do and take absolutely as little risk as possible. Sometimes they are forced into a side but that doesn't mean they want a side and when that happens they will try to manipulate it to their best advantage.

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The other 3 major sports have many more games each year with a more unpredictable bettor base than the NFL. A large swath of predictable amateurs populate the ranks of NFL bettors and for years books have shaded themselves toward the "less obvious" side when in doubt.

I'm not trying to give the impression that books are totalling bettor-losing money at a weighted (and unheard of) 57-58%, which including 10% vig results in an almost +20% disparity in profit. But 52-53% over time has benefitted them very nicely and provided added shareholder value over simply taking 10% on 50-50. Naturally this year books are getting killed by the same amateurs (graduated to novice now at least) who they've steadily leaned on for so long.