My guess is that this isn't a matter of common - but one of knowledge. 2 of the 4 big US carrier sell GSM phones with sim chips, but as the phones are almost all locked down, only tech folk know this.
To everyone else an AT&T phone is an AT&T phone, not a GSM phone, and no-one ever opens them up to pull out the SIM cards.
Whenever I've purchased a SIM card, I've gone to an AT&T store, and convinced the employee to look around for them. They always have them both pre and post-pay, but even AT&T employees don't necessarily know that till they look them up on the computer.