Does anyone know why Intel dual port network cards are designed for PCIe x4 slots, when a x1 slot ought to be enough?

Maybe so they have enough bandwidth when used on older pcie 1.0 boards, x1 would be on the limit.

I suppose that could be it, but PCIe v2 has been around for a long time. I did find a single-lane two-port card but it uses a Realtek chip.

It could also be because they leave room for protocol overhead? Or so that it is the same port for the quad port version of their NICs?

I suppose that could be it, but PCIe v2 has been around for a long time.

 

For GPU's yes, not so much for the other slots, only from Sandy Bridge on.