It's because standard have nightmarish level of complexity for something as simple as "pick a value from list one device provides and maybe change powering if that device signals you need to take less power"
That's what the PD standard basically is at core. It tucks on other things, too, like negotiating alternate modes, USB data roles, power roles, role swaps, etc. But the core of the standard is just "here's a list of values, and here's a way to tell the source which value I want to use" + method of communicating + some legacy USB stuff and some simplified alternative methods for power sources without PD chips (resistor pull-ups on CC lines to negotiate between legacy/1.5A/3.0A).