Why they don’t all have Apple Pay as an option, I do not know.
More to the point, that the web didn’t standardize UIs for payment and address information is probably a $100 billion mistake at least - in terms of what it costs everyone to do it their own way, and in lost time in having to constantly fill in the same form again and again.
It really is fast and easy[2] enough that it's become my default payment method in Poland, but honestly, prior to that, I kept going through the slightly more involved flow described in [0]. I really don't like giving my card details out, especially that at least three large e-commerce sites I did give those details to can pull money out of my account with zero confirmation or notice beyond me clicking the "buy!" button on the site, so I assume a criminal with access to that data could do it too (and it's a debit card, so I can't exactly do a chargeback).
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[0] - Or with one of the intermediaries[1] that you know handles it, which without mobile payments I'd be using anyway. The intermediary lets you turn what would be "give your card details to the merchant/payment processor" flow into "login to your bank directly through your bank's login page to just authorize a transfer request from one of your accounts" one. The nice thing about it is that a) it's faster (I remember my bank credentials, not my card number), b) the whole is is PUSH, not PULL - I send them money, c) it's not tied to a card; I can pay from cardless accounts, and d) the merchant doesn't get my card number or account details. The UI flow is simple too, there's a string of redirects and prefills that make a payment with the intermediary just a few clicks, a login, and some 2FA thrown in the middle.
[1] - Like these guys: https://en.wikipedia.org/wiki/PayU.
[2] - And safe! When you can get improvement on all three of the trade-off triangle, it only shows how much the older system sucks.