For the consumer GPUs, PCIe 4.0 x16 has plenty of BW headroom. The full sized x16 is more for stability reasons. Some vendors even put a couple of M.2 slots on PCI4/5 GPU board to recuperate the unused PCIe lanes.
Some low-end or middle range GPUs really only use 8 lanes, because that's how the chip is designed. Fewer lanes -> less silicon area for the pcie logic needed -> cheaper.
The chips which use 16 lanes take advantage of it and can saturate the link.