Second this. My bank does also splits the entry fields for the digits when there is a specific format, and it's a real PITA.
For those not on a ipad now, what happens is when trying to tap 5 digits in a row for 4 digit field: the keyboard disappears at the fourth tap, and the fith tap misses it's target and lands on the page.
If the 'somewhere' happens to be a link (i.e. the 'view on github' link on the left if right below the '4' when the keyboard is displayed) you get out of the page, and have to start over again. Trying to enter important information is stressful enough, getting out of the page every pack of digits and start over all again is a punishment in itself.
Good luck. Apple purposefully disables the ability to control where the keyboard is entering text. AFAIK there's no workaround (I’d love to know if there is!)
I gave up after the fourth extraneous tap.
So I guess "why aren't all credit card forms like this" is because people use iPads.
(In all seriousness, great idea but disappointing first impression.)