No but that may be because AC games attempts to maintain some lose hold on realism.
Yup, jumping hundreds of meters into hay, flying on wind currents created by bonfires and time travel establish AC's firm grip on realism. Also, the question was how does AC justify its sandboxy-ness?
Technically, no time travel. But you can get in a fight with the pope and beat him all over the floor.
You can also beat up illegit husbands on request from their sisters. Collect gear for your character, armor, and unravel mysteries.
And in Brotherhood purchase the world.
Oh, and they're about to do deus Ex on the stream.