> Why can't we have both multiple staircases and fire-retardant construction?

Because that

1) that costs more money (if you object, "How dare we value money more than human lives?", replace "costs money" with "requires a greater amount of the limited quantity of human labor"), which means fewer homes are built, which means it costs more to purchase/rent a home.

2) Means buildings have to deal with the inherent topological constraints of double-loaded corridors (ie, apartments can't "cross-cut" the building, allowing cross-ventilation/greater number of windows.