This was my exact thought process as well. If they had an overlap year or 2 of USB-c cables with charging bricks, or included a USB-c to USB-a adapter in the box like when the iP7 came out and they included a lighting to 3.5 adapter it would make more sense. But I for one didn’t have a computer or a charging brick that used USB-c when they made that change. Actually I still didn’t until this week when I got a new Mac Mini. My PC and all my charging bricks use USB-a only. And my bought separately wireless chargers don’t count because again, I had to buy them separately.I'm not even convinced it actually cut down waste. They switched the included cable to USB-C -> Lightning rather than USB-A, and many people don't have extra USB-C charging bricks lying around. That resulted in a lot of people now having to buy separate chargers with their iPhones, and those of course came in their own packaging and generated more waste (and emissions from transportation) than just packaging it with the phone would have.
I'm not even quite sure why they did it. The charger can't have been expensive to manufacture and there isn't exactly a huge opportunity cost to it either (not everyone is going to buy the official Apple branded charger). It just doesn't make much sense to me.
It’s nothing but cost cutting for apple. Plain and simple. Anyone who thinks otherwise is a corporate greed/greenwashing sympathizer.