Why can't I just connect HDMI to air 2 pro?

I would like to understand the process or the logic behind the adapters.
Let’s say I want to connect a laptop and I have an HDMI cable, I can connect the HDMI cable to my laptop just like if it was any other monitor and then connect an HDMI → USB C adapter to the other end of the cable.
Why does it not show any image in the glasses?
Why does using just the USB-C cable works?

Please consider the power supply when using our glass, as it requires power from the USB-C cable instead of a separate power cable used by other physical monitors. Note that most HDMI adaptors do not support power transmission.

I am using a dongle that is receiving power, and my glasses are dimming (which I think is bcs they are receiving power) but I can’t see any image.

Did you see the external monitor in the Display setting?