The power pins are in the spec rated for 9.2A each, for a total of 662.4W@12V. To be compliant the connector must handle at least that. At the same time, the max advertised capacity is 600W, so a device must pull at most that.
So by the spec it must have at 10% headroom - and to live up to that, manufacturers must add their own margin on top. Rating for 1kW would be a 66% margin and a humongous waste of copper, and it wouldn't solve this contact issue in the first place.
Regarding wire gauge - unrelated to the problem at hand - 16AWG is already overkill. For house installation in EU, we rate that gauge for 13A to 18.5A depending on thermal conditons, with 13A being "very poor" thermal conditions (no cooling and surrounded by other hot things). With 6 pairs in parallel, that gives a conservative minimum of 78A capacity - and with the excellent thermal conditions of a case with fans ("very good" is normally just a dangling cable, not one with fans cooling it), the realistic capacity should exceed 120A. As you say, we need only 50A, so that's a 140% margin! Even 20AWG would be plenty here.
So by the spec it must have at 10% headroom - and to live up to that, manufacturers must add their own margin on top. Rating for 1kW would be a 66% margin and a humongous waste of copper, and it wouldn't solve this contact issue in the first place.
Regarding wire gauge - unrelated to the problem at hand - 16AWG is already overkill. For house installation in EU, we rate that gauge for 13A to 18.5A depending on thermal conditons, with 13A being "very poor" thermal conditions (no cooling and surrounded by other hot things). With 6 pairs in parallel, that gives a conservative minimum of 78A capacity - and with the excellent thermal conditions of a case with fans ("very good" is normally just a dangling cable, not one with fans cooling it), the realistic capacity should exceed 120A. As you say, we need only 50A, so that's a 140% margin! Even 20AWG would be plenty here.