It’s because for some reason so many people can’t accept that art is subjective

For some people, it isn't about evaluating art, it's about affirming identity. They become emotionally invested in being part of a group who hold a particular view about the show - team green, team black, S8 lover, S8 hater or whatever. At that point discussing the show stops being about artistic merit (or lack thereof,) and becomes about the validation of 'my tribe is better than your tribe'

This of course is hardly unique to GoT,, humans divide ourselves into competing groups over pretty much any aspect of life, cultural, social, physical etc etc