Because reviews are pretty much always by a stranger to me, I have no way of valuing their expression. Those people aren't a part of my life and there's no way change that. It would make more sense to me to establish a relationship with people before reading their reviews of life as they see it.
Yeah, generally speaking single reviews from people are generally not terribly useful as someone could love a game to the ends of the earth and rave about it but it is not even a type of game that I personally would ever give the time of day to. I've played some games that have high ratings and people go ape shit over allegedly which I ended up thinking "what the hell is this crap?" and wondering if everyone is completely mentally insane or what. :) A review on its own is one person's perspective as it is at a single instance in time as you say, and that could be influenced by a number of factors including their spouse/partner bitching at them for not taking out the garbage or whatever, we just have no context when reading them as to what the person was feeling at the time and why.
I've seen people give the lowest numerical/star rating to some game just because it didn't contain support for their native language of Swahili or some other non-mainstream language that zero games have ever supported for example. A game not supporting a language that no game has ever supported is totally not even remotely a good reason to rate it as low as possible. That's a bogus useless emotionally dysfunctional rating IMHO, but such types of ratings are out there.
Or maybe the website was under intense pressure during a sale and the person had difficulty completing their purchase or downloading the game, so they have this vengeance chip on their shoulder with "oh yeah, I'll show you!" on their minds and they go give the game a piss rating when they haven't even played it yet and the problem they had that they're having an emotional breakdown and tantrum over had nothing at all to do with the game, the game publisher or developer whatsoever.
I do read some individual reviews (both good and bad, mainly on Steam not here) to get some ideas about what I might expect from a game, and look at the game's aggregate ratings (again usually on Steam and/or Metacritic user ratings) to get a general idea, then couple that with trailers and maybe a bit of gameplay video footage etc. That seems to be most useful.
To me though the least useful ratings tend to be the ones that have the highest amount of emotion conveyed in them, both good and bad. The bad ones because they're usually very negative and sarcastic and fail to be genuinely objective usually, and the good ones because they tend to come off like fanboy praise and equally non-objective. The best reviews IMHO are the ones that convey the absolute least amount of emotional content and the highest number of objective comments with examples. Unfortunately, truly objective reviews seem to be rather uncommon.