While it would be cool to have reactivity for backgrounds, it just really isn't feasible with the sheer number of backgrounds available (13). They also already have reactivity for Race, Class, Origin characters, Subclass, and Deity Selection; adding in backgrounds would just be a little too much work. Like even BG3's competitors such as the owlcat pathfinder games don't have background reactivity because of how many possible permutations you would need.
IIRC though, there actually is a little bit of "background" reactivity as all custom characters are supposed to be from the city of Baldur's Gate and so they have the "Balduran" tag which lets them know things about the city. Other characters like Gale (who is from Waterdeep) don't have this tag. I think this and the unique background inspiration point quests are probably the most we're going to get in this area.
