They can, but it turns out that the handoff points can themselves be more problematic than the effects of working for 28 hours. They did lots of published research which showed that, which is why this policy change is even happening.
Also, a 28 hour shift doesn't mean that the doctor is working (or even awake) the entire time - they're on-shift, but there are periods of downtime. It's not necessarily 28 hours of constant activity.