It is quite simply a way for manufacturers to save money. And absolutely nothing to do with "saving the environment". Look at the Pixel 6: to get the full fast charge speed you have to buy a certain charger so you can't just use your old one (I had a Qualcomm Quick Charge 3.0 charger before I got my Pixel which doesn't charge fast on the Pixel), meaning you need to buy a charger anyway which is WORSE for the environment as it means 2 packages getting shipped to your house.
To be fair, Qualcomm's QC is usually limited to SD devices and you have to pay royalty on top. USB-PD is the standard Pixels uses which was created by the group behind the USB standard, although it did come late to the party, hence why the fast charging scene is such a mess. Luckily a few other fast charging is also making their chargers USB-PD compatible so even though it's slow, we're eventually getting to the point everything will eventually faster charge other devices. Qualcomm did announce QC 4/4+ with USB-PD compatibility but for some reason it wasn't popular with manufacturers (most likely due to them going with their own fast charge tech).