There are reports and rumours that NVIDIA are forcing the data centre companies to buy the GPUs upfront. Though there isn't a lot of clear evidence that this is the case.

[1] https://tech.yahoo.com/gaming/articles/did-nvidia-manufactur...

[2] https://www.reddit.com/r/Games/comments/1xdjbd1/multiple_rep...

[3] https://www.techradar.com/computing/gpu/is-the-gpu-stock-cri...

[4] https://www.businessinsider.com/amazon-nvidia-ai-gpu-data-ce...