Why would the sandbox ever be non-reactive, except as the result of limitations in the programming code* for a CRPG? I understand why linear campaigns may need to negate player choice to reach a desired, prescripted end point on the adventure path, but why would a sandbox setting ever not be allowed to develop/change in response to player activity?
Because the GM isn't interested in changing it? Maybe because despite being given a chance the change things, the players don't choose to or simply haven't done things that change it? Maybe because the players are having a grand old time kicking in doors, killing monsters, and taking their stuff, and aren't interested in what happens after they've left the dungeon behind, so why bother? I'm sure there are other reasons - some good for the group as a whole, others not.
My experience is that the nature of the campaign experience has less to do with what model you're theoretically following, and has more to do with how the GM and players choose to work within the model.