Ohm's Law and Joule's First Law
More current through a given conductor = more heat generated, eventually to the point it becomes too hot to handle and begins melting the insulation. This is how every electrical cable and connector on the planet works; it's physics.
The SAE Combo connector has big, dedicated DC pins with plenty of space between them so that the risk of overheating and arcing is reduced. That's precisely why it looks the way it does.
Tesla claim they have "successfully tested their connector up to 900 A without liquid cooling" and I'd want to know by what measure it "succeeded". Didn't melt in <5 minutes?
Newer "V3" Tesla Superchargers use liquid-cooled cords; they have to to keep the cable anywhere near as small without overheating.