When I look at a product review, I often want to skim it. I may only care if the Foo has feature Bar, but not want to hear about the rest of it. LWN's reviews are pretty good and I usually read the whole thing. But still, some days I have more time than others. That time may also come in small chunks while waiting for something to compile or finish. With a video, I have to listen to the whole thing.
What if I want use Google to search for a product review? Google is pretty good at searching text, but can't search video at all.
If there's some aspect of the product that really is best described by a video, then sure, it's nice to see a video. Having the whole thing be a video is like having the whole thing be a giant picture. At the end of the day, text is still the best at doing the things that text can do.