Why the FUCK can I not get one S rank

The only appropriate response here is: "Mad cuz bad"

Look at it the other way. Riot grading is flawed and frequently conflicts with good play in real situations. Don't stress about it.

op.gg also grades for games. Their system seems to be a bit more in-tune with what wins games.

an S is grade/rating of PERFECT if you dont play PERFECTLY then you dont get the S

it is based and calculated on the champ you play in a certain role vs all the over players who have played that same champ in that same role

for example yas mid

if you dont hit #s that are in the top 5% you arent getting an S.

and this is based on your Kayn game you went 12-2-7 you place 6 wards, bought no control wards, destroyed no wards. so based on your kda, damage, damage to objectives , cs im gonna say this is the only reason you didnt get an S in this game.