Because they know that people don't always use the chargers that came with their devices. People buy extras so they can have one at home and one at work. They borrow chargers from friends. Sometimes, they even plug their phones into their computers to charge.
If you plug a devices that was designed around the USB Power Delivery spec into a charger that was designed around a proprietary rapid charging standard, (or vice versa) the two devices aren't going to understand each-other correctly during the process of negotiating charge voltage, current limiting, and which data lines (if any) to use to supplement charge current.
It would REALLY suck your USB standard phone signaled the charger that it wants 5V@3A and the charger, which uses a proprietary charge protocol, misunderstood the message and not only upped VBUS to 20V, but put 20V across all of the data pairs as well.
If that were to happen and the phone were to be irreparably damaged, the blame is going to get spread around between the charger and phone manufacturers. And some people are going to look at the situation and switch to iPhones because "at least they don't have this problem."