the only reason I can think of is because they have to pass audio through the HDMI port but then there is HDMI drivers too

are those sound drivers used for something else?

Do you mean video card driver or a driver suits ? The latter installs more than just the video card driver but although other chip-set drivers (eg motherboard, audio etc.?).

Some AMD cards at least have 3D audio processing included.

the only reason I can think of is because they have to pass audio through the HDMI port but then there is HDMI drivers too

are those sound drivers used for something else?

I think that is the reason. Since HDMI is able to transport audio data there need to be an driver driver for this.

HDMI.... Connect your PC to your TV over HDMI and behold: The TV also plays the Audio.

So yes, since GPUs included HDMI, they have had a need for Audio drivers. IDK if "HDMI Drivers" would also take care of the audio part, but I kinda doubt it.

Yep, same as above. HDMI is basically scart 2.0 :)

DisplayPort also carries audio (along with its laptop friendly mini-DisplayPort counterpart).

Though, to be fair, I haven't found very many DisplayPort monitors with speakers built in.