House edition

Theres two answers
First is the sad truth that they often did sag, and it was a pain in the fucking ass to replace, but the wood was strong enough to last a long ass time before it needed to.
Second, the more interesting answer, is that on walls that are jettied (overhang, like in pic rel) the floor is actually more likely to bow than sag. This is because the beams are propped up over the wall beneath them, but the wall above them sits at the end of the beam, so the edges are pushed downward with the full weight of the floors and roof above and the middle of the floor is strengthened.
A stone building would either be narrower than the wood ones or their would be a wall(s) down the middle acting as a support.