The new iMac uses a 143W power adapter. Apple couldn't reduce M1 power to 100W to charge it using USB-C PD.
It's basically just a Mac Mini but with a screen, keyboard and mouse/trackpad and a monitor. Should be under 100W easy. There's certainly another reason for it. Maybe just to get rid of an enormous stock of power bricks (so much for environment)... I do like the ethernet port on the power brick though, hope they put that on the next MacBook Pro.
The new iPad Pro still includes a power adapter in the box. Only iPhone team cares about environment, but wait, they are still using Lightning instead of USB-C.