Why do dual GPU cards only need 2 cables?

[MR.B]

That's not quite true. 75W 6pin divided by 12V = 6.25 A, split over 3 wires is 2.08 A per wire/pin..I don't think that's really pushing the wire/connector, More likely the PCB tracks
I still feel they do it for the reasons in My above post also 2 connectors could help with any shortfall if the PSU has separate 12V rails

No they do do it because of what i said. It's not about if the cable can provide enough power, it's more about stable consistent power, especially on cheap PSU's. I have experience with building computers that used cheap PSU's and they can shorten the life of components from poor efficiency and under voltage/over voltage.
ATI and NV have even said this in the past. One of the reasons 8pin come out is because it can provide more stable power over 6pin, just like 6pin could provide more stable power over the old 4pin molex connectors used in the past. It's always a good idea to use 2 connectors on high end cards.

 

[Jakus]

I take all Your points but would still argue it's not the connectors that are important it's the thing on the other end of the wire that's important the "Power Supply" a crap PSU with 4x 8pin connections to the Card will never help anything ! also understand ATI/Nvidia's angle on the issue ( just very surprised they allow their sub vender's to supply adapter leads with their cards

Totally agree about cheap Power Supplies though and I have seen quality units literately burn up and die protecting expensive components rather than allow an overvolt situation....You can generally grade PSU's by the weight without even reading the label's