Why don't the sports books just move the line?

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What's the reasoning behind sites giving, let's say -3 at -150, instead of just moving the line to a higher number? They're basically saying they think the -3 is too low, so why don't they just up the line? Is it because some bettors will see a line they think is too low and bet on it anyways despite the 50% juice? And the high juice is more profitable to the sports book than moving the line?

Sorry for what may be a newb question, but alas, I am new to sports betting.