Well, because in that case it is a) a sort of a minimum acceptance testing, i.e. if they cannot even convince those guys it probably really really should not be released yet, and b) simly marketing, because the youtubers will provide great free-of-charge PR for them.
This sounds awfully negative towards these youtubers - actually I do respect them and value their opinion, especially if they have a couple of days to play around with it on their own PC, because my thinking is that what they say is likely nearer to what I would say when having played around with the 8K for a week or so, than what Ben or Sjef would be saying.
I would not be looking hard to find the smallest flaws - after an initial, quick check for distortions, sweetspot, ghosting etc. I would be looking to experience the 8K with a positive attitude towards it, being curious how it enhances my experience above the Rift and Vive. Only if flaws disturb me in my experience would I be unhappy with them. The Ben’s and Sjef’s might be annoyed even by imperfections which insult them as tekkies. So they may then find it harder to ignore such flaws than I would find it.
This is the reason I am not going to consider such beta test, even if only performed by the happy puppies, useless.