alt Hacker NewsMain problem seems to be they're kinda badly utilized (IMHO) on many motherboards. Most seem to go with two x16 slots so you get x8 lanes in both.
There are some exceptions, but I haven't seen one with for example four x16 slots that support PCIe 5.0 x4 lanes with bifurcation.
You can buy add-in cards that do lane bifurcation
E.g. https://www.ebay.co.uk/itm/126656188922
Most motherboards don’t go beyond 2x8 with 2x16 physical slots because there is little actual use for it and it costs quite a bit of money.