blocking phone calls on iPhone

Because it doesn't. True call blocking can only be done by the carrier.

No third party app could do this without compromising the security of the phone. That's one of the reasons the API calls necessary to do it are not public.

As I said, call blocking can only truly be done at the carrier level.  The best your phone could do would be to reject the call and send it to voicemail. That's not blocking. It may appear the same to you, except for the fact that the caller can leave a voice mail, but it's not actually blocking anything. The call has to get to your phone before it can be rejected.

Thanks, I appreciate the response.

I'm not really concerned whether the iPhone blocks the call or simply rejects it. Is there any reason that Apple doesn't allow me to reject phone calls? (The best solution I can find otherwise is simply to give the unwanted caller a silent ring, which isn't really the same thing.)

Mine does give me an option to reject or you can double tap the sleep button on top of the iPhone.

True, you can reject calls manually or by double-tapping the sleep button. I'm wondering why there isn't a feature that simply allows you to automatically reject calls from certain numbers so that the user doesn't need to make the effort to see who's calling and then reject the call. A feature like that would seem pretty useful for dealing with phone spam ... and it's something that appears available on competitors' phones.

IMO, the best call blocker (for me) is me... I don't answer calls I don't recognize.  It's pretty foolproof.