There are other pieces of the hardware that developers don't have direct access to, and no API to get to them. It is not a novel or unusual idea that some parts of the hardware are simply inaccessible to the outside world.
Before there was NFC payment, there was (and continues to be) VISA / MasterCard / Discover / Amex. Why didn't large banks make another card? They could have. But it costs too much.
Apple spent years developing Apple Pay. Yes, I know NFC (as a tech) is a separate thing. But Apple spent years testing it, coming up with how it would work, and making it seamless.
The demands will not stop with "mobile wallet makers" getting access to NFC. They will also demand that any API open to third-parties should operate just as easily as the Apple Wallet. And then some banks will move their cards to their own wallets... then we'll have like half-a-dozen "wallet" apps running on my phone, with varying levels of consumer protection, etc.
Here's the thing... no user is clamoring for a better wallet. Apple Wallet works fine. No user is demanding choice in the wallet arena (and if you say you are, you're lying). This is something completely manufactured by big banks, and if Apple is forced to open it up, it will make the iPhone worse, and less secure. Guaranteed.