It's not that simple.
USB-C uses communication between the power supply and the device to establish the power supplies capabilities, and without that communication, the device will assume it's only being powered from a basic 5V supply.

The Pi5 documentation covers what the Pi5 expects - https://www.raspberrypi.com/document...raspberry-pi-5

However I'm surprised there's no mention about powering it via the GPIO header, and my initial search hasn't revealed any mention on whether you could simply connect 5V to the GPIO?