Brand is actually a very important factor on how people view something.
Brand can not only inflate a game score, but also its sales, as for example, people wouldn't even be talking about something like 1-2 Switch, if it wasn't made by Nintendo.

Now, there are more to it than just brand, Zelda as in your example, is a very, very beloved series, and it has been constantly delivering great games from what anyone can tell, but I have to agree, because Skyward Sword, which was a quite criticized game, still managed to score like Uncharted 4, which was a widely, and very strongly praised game, which is also from a very famous brand, by the way.

I don't think its nostalgia the point here, but that Nintendo games are reviewed on different standards, like indie games also are reviewed on different standards, and Zelda is also reviewed on a different standard by itself just as well, because of how good its track record is, it intimidates and pressures people to flock on the collective and also usual pattern.

I don't know if BotW will be a good game or not, it looks like it is, but I agree, if the exact same game by another developer was released, it wouldn't score nowhere near what I expect BotW to score, which doesn't mean it doesn't deserve it, again, I still don't know how good will it be, but that actual criticism would be way more present.

There are lots of different situations that have influency on game scores outside of just the game itself, and when we compare them all by a single crude number to see which is higher, lots of them will seem unfair, because of the different standards they are judged by.