Because the 140W charging adapter supports USB-C EPR (extended power range) [0], which allows compatible USB-C chargers, cables, and devices to work at up to 240W. (Obviously Apple's 140W charger only supports up to 140W.) However, the spec is so new that there's currently no USB-C to USB-C cable that supports 140W. I don't think we know yet if the MacBooks can charge at 140W with an EPR USB-C cable, not least because nobody is selling that cable yet. But I wouldn't be surprised if it doesn't work because that'd require all three USB-C ports to support it.)
The MagSafe 3 cable goes from 140W USB-C to MagSafe, but the USB-C charger is 100% standards-compliant USB-C EPR.
140W charging on the 16" had me worried that Apple was using a proprietary/non-conforming extension to USB-C, just like Nintendo did for the Switch. The reports of bricked Switches from standard USB-C chargers was an unfortunate demonstration of why this is a bad idea, and why it's important that Apple is using a standard (even if it's very new) method of increasing power delivery.
I really doubt that Apple would have gone out of their way to invent their own power profiles for the 16-inch MacBook Pro if one didn't already exist.
The 16-inch MacBook Pro certainly doesn't need 140 W to run. The Intel i9 MacBook Pro comes with a "measly" 96 W charger and it tops up just fine. The 140 W is there for fast charging and if they had to stick to 100 W, they could've made a professional machine just fine. It's not like the most common complaint about the MacBooks between 2016 and 2021 was "it charges up too slow".
There’s no real problem making a charger than can deliver more than the spec allows, but you wouldn’t be able to use it with a C-C cable. This isn’t one.
