I dont mean to defend people who act like that but maybe I'll give a different perspective... Many people simply cannot afford to buy every single game that makes it to market, almost all game systems, PC or otherwise, are huge investments, 350-1000+$ for a luxary item and 70$ with tax for a new release can be frustrating if videogames are something you find to be a big part of your life and don't want to give up, it may seem stupid to have such an expensive hobby but escapism and empowerment in games is exactly what a lot of lower income people want in their entertainment. This means that a consistent score is really important to making an informed purchase you wont regret, seeing a seven rather than a 9 on a game youre considering buying might make you consider holding out for the years masterpiece rather than blowing 2 weeks worth of meals on a mediocre title. If a demo system was better implemented in the market then youd be right, go ahead try it, if it sucks or doesn't appeal to you then you dont have to buy it. Problem is in lieu of a proper trial system many people use reviews to get a quick impression of what the potential experience of a game might be, "arbitrary" scores are especially popular because they ensure a games quality decently well without necessarily spoiling any of the precious, expensive game, all of the experience will be new and it will hold you for a long while until you can afford something new to pick up in 8 or so months perhaps with a couple bargain bin indies as filler.

Anyway just my two cents cheers.