Because as it’s currently designed Apple is not handling any of the actual transactions- those are handled by the Payment Service Provider that the merchant is required to provide to be granted Apple’s permission to use the API.
If Apple opened it up to non-Merchants (who don’t come with their own PSP), then Apple would have to act as the PSP which is a much larger headache that they don’t want to deal with.
Because as it’s currently designed Apple is not handling any of the actual transactions- those are handled by the Payment Service Provider that the merchant is required to provide to be granted Apple’s permission to use the API.
If Apple opened it up to non-Merchants (who don’t come with their own PSP), then Apple would have to act as the PSP which is a much larger headache that they don’t want to deal with.