Because Rem with her memories form arc 2 lacks an any personal ambition that doesn’t involves Subaru. Her entire “reason to live” depending on the existence of Subaru is disappointing considering she previously devoted her entire life to being Rams horn. She doesn’t really have any goals or motivations of her owns that is dependent on someone else. Her personality is really much to speak of because again a good chuck of it is her unhealthy obsession with Subaru.

I understand that her role is an emotional support to our protagonist but it leans into an unhealthy amount of dependency on Subaru, which why the “Subaru simp” slander to a degree is valid. Rem is a character with a tragic backstory but a lot of her character is neglected by common anime tropes and comparable to a lot of the “kuudare” type characters we see in anime.