My guess would be that's due to so many people reviewing products that they have won in giveaways etc and usually tend to give overly positive reviews, sometimes after using a product only for a week or so - not really possible to see the effects that quickly.
I only have a couple of holy grails personally, that I have used at least for months continuously and I'm convinced they do wonders for my skin.
But each to their own, some people are naturally more positive and everything's amazing. I'm a sceptical person and I need to be absolutely convinced because there's so much misinformation that the best we can do for each other, and generally online, is - be honest.