What makes you think that Apple is incapable of "forgetting" something and just chooses to exclude a feature?
That's easy - because every one of their products goes through extensive design and evaluation processes, led by what is widely acknowledged as one of the world's best industrial design teams which has won awards for their work, and then tested and tested and tested to fine tune the final device.
There's no way to know for sure that they didn't 'just forget' something, but there is every reason to believe, from their track record and product development history that it is very unlikely that anything missing from the iPhone is missing for any reason other than that they intentionally excluded it.
There could be several possible reasons for excluding what seems like a perfectly good feature - it may be that adding that feature detracted from what they saw as the form and function of the device, or that adding it caused undue battery drain, or conflicted with licensing or the deal with AT&T. It may have been that they left features out in order to get the item to market, with the intention of adding them later. These are things we don't know. What we DO know is how the company typically functions when creating a new product, and there's nothing to suggest they 'forget' things.
There IS, on the other hand, plenty of evidence to suggest they sometimes get things wrong!