"We're going to be environmentally friendly by not including the wall wart because everyone has those already. But we're not going to include a cable that fits in any of the ones you have..."
Now, that said, I've found that USB-C PD is far faster and better for charging things, and I avoid plugging into USB-A when I can. So I think it's really a bit more of the old Apple "we're not going to give them arrow keys because we want them to learn how to use the mouse" kind of philosophy. With the added bonus, of course, that they're saving money and making more profit.