You're missing the point GP made though. He wants to replace one good card with 4 worse ones. It's not like that rack has 3 or 7 additional slots just unused. They're also already taken by the setup. And in the link you provided 5 out of 6 offerings are still Nvidia GPUs.
Of course data centers are happy to oblige to customer demands, but initial cost per GPU and performance per watt are certainly not the only relevant factors.
For anything datacenter related, customers are very sensitive to price per performance. And datacenters are happy to oblige.