It's not advisable to build a mining rig right now, GPUS are out of stock in every store in my country and that's not the worst part, I bought my gtx1660 ti for 400$ each and today it's 640$, the fact is people even still don't mind buying this cards at 640$ each, are they stupid? I don't know but getting back ROI should be their most worries, I think it's now nothing but FOMO
As your title says, FOMO. Miners or new miners won't mind if they'll be paying with a premium for each card that they'll get. Why? because the market is in good shape and it doesn't matter to them because ROI can be faster this time.
Those who buy cards without proper knowledge are the ones that are at the most risk. Because they just got into mining because of hype.