Because anyone bothering to make a written or video review of something like this is (a) a spoiled brat, and (b) inclined towards outrageous hyperbole, because that's where the clicks and the likes and they funnies are.
The old SD consoles and portables don't sound great, but they play OK and at full speed. As you say, a marvel, especially at the near-nothing price, with legitimate licensing from the Sega (and Mortal Kombat) rights holders, and in wide distribution (no need to order sketchy stuff from China). Maybe it's an age thing -- I know that as an "older person," (genX) I still think miniaturization is still pretty amazing.
Putting my two cents in, I think that this product (Sega Genesis HD) has the chance to be pretty cool. I, too, am a Gen Xer and think miniturization in all forms (whether it be products by AtGames or Emulation type devices) is quite amazing and really something spectacular to occur. True, they (most likely) are not going to be perfect replicas of the original hardware and/or software; but getting darn close to it is still pretty cool. I really do hope that the next Genesis HDs that are shipped out and reviewed can live up to the hype and be a good, reasonable, and cheap alternative to going out and getting the original items themselves.