A:AnswerMost likely because there’s very little margin on Apple products so they’d prefer you to order with a plan where they make some money or pay extra if you buy the phone yourself. Just ask for a price match though.
A:AnswerBecause Best Buy was trying to charge an additional $100 to get an unlocked phone that wasn't tied to any carrier. They got a lot of heat for that, and have since forced you to lock your device to a carrier. (https://www.theverge.com/circuitbreaker/2017/10/31/16585758/best-buy-stops-selling-full-price-iphone-x-iphone-8)
A:AnswerIt's an incentive to do the monthly payment. You can do a down payment at time of purchase, then pay off the rest through Verizon. I.e. If it is $699, do a down payment of $675, then your monthly payment on your bill will be $1, or you can call Verizon and pay the $24.