ZW____ :
why is Arock z270 supercarrier able to support 4-way SLI/crossfire while i7 7700k only has 16pcie lanes?
Because the Chipset has an additional 24 PCIe lanes used for other devices such as the M.2 drives, sata ports, etc.
In this case the 16 pcie lanes of the CPU are split amongst the first 2 slots, and 16 additional are split from the motherboard PCH count of 24 into 2 slots. Hence you get 4 way SLI with 4 x8 lanes.