Graphics card power supply question

Probably a dumb question, but I'm looking at the 2080ti specs page on Nvidia's website, and it says that it needs two 8-pin connectors even though it only has a power requirement of 250W. Theoretically, a 6 pin + 8 pin power connector would be able to supply that, since that setup provides 300W. Why is it that the card requires the 2x8 pin connectors still?

Even though I'm probably not going to get the 2080ti, I'm asking the question because my power supply only has two 6 pin connectors, and theoretically with a 6 to 8 pin adapter, I would still have enough power to supply many of the graphics cards that require a 8 pin connector. For example, the 2070 super has a requirement of 215W, while two 6 pin connectors is able to provide 225W, so with a 6 to 8 adapter it should be able to power it, right?