Because old cities had these kinds of fires constantly (in my city there's still a paiting of a fire from 18th century that destroyed half the Old Town) and usu

Because old cities had these kinds of fires constantly (in my city there's still a paiting of a fire from 18th century that destroyed half the Old Town) and usually only stone/brick buildings survived. This isn't a 0%/100% thing, but it increases the chances by a big margin.