I believe PCIe 4.0 wasnt that useful for big server farms, because network cards were already at 400gbps. Even at 100gbps networking, that's only 2 ports.
PCIe 5.0 is only 1 port of 400gbps.
So PCIe 6.0 is the next actually big step for a lot of servers, so you can finally get dual 400gbps ports on 1 card