Every single well positioned brand gets favored treatment, that's just how life works, every Sony game do too, that's what keeping quality over time rewards, it's not like people feel obligated to put scores to games they don't like, it's that game are great consistently and keep surpassing expectations.
Other franchises have more varied scores because they're not consistent or because while good, are not at the same level.
A mainline Zelda release is like the Olympics or FIFA World Cup level of event for the industry, it's even more symbolic for most Nintendo fans than a Mario game, you don't get that status because of some social pressure our magic name, you get it by keeping and increasing quality over Time.
I'm not exactly a Zelda fan and am still yet to play a below average, bad or even not great Zelda game.
Yes, Zelda doesn't get scores for being Zelda, it gets scores for being good, very VERY good.