Mrs. KRSW's work Dell Latitude laptop also has the Dell-proprietary barrel DC plug AND support USB-C. Why? Likely it's cost. Considering you can find a replacement Dell charger on Amazon for as cheap as $7 and USB-C charger+cord equivalents are $30+, this seems to be the obvious answer. The DC bricks are just pumping out 19vDC all day long, no USB-C negotiation circuitry needed. No real risk of consumers blowing up their devices by plugging their phone or other devices into it.