Efficient DC-DC converters are area consuming due to high voltage devices abd the capacitors in general being very large. You would need SMD inductors on top of

Efficient DC-DC converters are area consuming due to high voltage devices abd the capacitors in general being very large. You would need SMD inductors on top of that. This means you used very precious silicon area for power delivery. It creates a lot of reliability headache since higher voltages cannot be routed closer to low-voltage stuff. If you're not crazy high in current consumption density (A/mm2) it doesn't make sense. They also have another design advantage here. I believe they're not IO limited with their bumps.So they can use most of it for power, which is much better than any DC-DC solution.