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.

[0]: https://www.reddit.com/r/UsbCHardware/comments/qat7ej/new_ap...