Three sportsbooks open. Take the side you already like and look at the price on all three. The worst that happens is they're tied. The best that happens is one is sitting half a point off the consensus number. That half point, on the right side of three or seven in football, is worth roughly twenty cents in vig over time.
Multiply that by the number of bets in a season. The number is large. The number is also boring — which is why it doesn't get done. Line-shopping is the only edge in this entire ecosystem that requires zero analytical skill and is available to everyone. The reason most players don't capture it is that the work is small and constant, and there is no story to tell when it works.