The DisplayPort connector includes a supply voltage. While it is weaker than in USB, it is strong enough to provide power to an active DisplayPort to HDMI converter, which can have the appearance of a video cable that can connect a DisplayPort source to an HDMI sink.

On one of the HDMI pins there is a DC voltage, but it has other purposes and it is too weak to provide power for a video converter.

This is why an HDMI to DisplayPort converter always requires an additional external power supply.