Why do stores have to individually roll out Apple pay in the US ? How does that work?

Virtually every card reader out there that fairly new already have the chip/NFC reader also built in. I am not sure on the specifics of unlocking that bit, but many stores lock it because they want you to use their payment system, like WalMart, they have WalMart pay or whatever it is called in their app, Target has their Red Card. I’m not sure if there is extra cost on the banking side for the stores to have that form of payment, but that may also play a role.