Here the thing, your opinion isn't fact (nor anyone else's). Fuck I hate when someone says a game is objectively better than another (even if it's a game I really like). EVERYONE's opinion or review on a game is subjective and the quality of a game is subjective (unless it's some horribly broken). I couldn't even finish Astral Chain or Sekiro because I got bored with them, yet I played through BOTW like 4 times. And Days Gone is one of my favorite open world games this gen. I just started Death Stranding last night and I'm loving just like I did MGSV (which was my personal GOTY that year). Again all that is subjective and just my opinion and I wouldn't dare suggest that my opinion is fact because I'm not that arrogant. To put it simply, people like different shit (dunno why it's so hard to get that into people's head).
Okay, sorry. I used the wrong word here. objectively is maybe an exaggeration (that's why I put IMHO .at the end of the sentence.) You happy?
Now, still, there are some objective factors which make Death Stranding an excellent game (I gave it 8.6/10 which is a very high score from me), but not game of the year. Level design, AI behaviour and variety of gameplay are objective factors for example. The enemy AI was rather bad in the "war" sections for example, besides some serious enemy rush, I hardly had to do anything else, than spray bullets at them.
Combine this with the extremely linear level design of those parts and we have a rather average shooter here. Same for the fight against human: they were extremely stupid to begin with and hand-to-hand combat is a joke. (Non-killing weapons are creative and fun to use tho, just too easy to use them because of brain dead AI.)
I don't have to say, how gameplay variety is missing - if we think beyond the "try to walk and "survive" in the world outside " element which is indeed revolutionary.
Now, don't get me wrong, I LOVE Death Standing, I really do, because of its revolutionary ideas, its artful, deep story (best story this year, and by far), ambience and graphics (just to name a few things) but the - kind of objective factors - can't be ignored. Funny you mentioned, Days Gone was a better game if we put those together gameplaywise, but the story was better in Death Stranding. But those are still "objective" values that we have to compare if we want to choose the best game this year.
I hope that you get my point with all that.
Now, the OVERALL judgement is still subjective - you are right on that - since when I have to give a final score, I may value the almost 10/10 story above the extremely repetitive (but still revolutionary and fun) gameplay, the weak AI, etc which are still there, but as an OVERALL impression, my judgement is of course subjective.
If Death Stranding were a movie, it would easily win the Oscar for this year as it's a work of art. But as a game, it have to compete against other excellent games.