- It's quite possible she doesn't know or believe that he might actually love her. She probably assumes she's only being used just as she's using him, as per their intentions in "I'd Be Surprisingly Good For You." It's not until she's dying that she realizes that yes, the main reason why he's stuck by her all this time is because he really must love her, hence the song.
Headscratchers / Evita
In the film, why does Eva lock Perón out of her room after "Don't Cry For Me Argentina"? Are we meant to assume she has no genuine feelings for him at this point (although her reaction to his arrest in "A New Argentina" suggests otherwise) and she's just playing his adoring wife for the public)?