So, for example: Bathrooms. While there may have been one or two [sets of] bathrooms per section of floor in commercial use, and this worked fine, residential tenants want at least one full bathroom all for themselves in their relatively small 1, 2, or 3-bedroom spread.
Or kitchens: What might have been a few limited break-room-type kitchenettes per floor needs to be a full kitchen, per-unit.
Those pipes need to live somewhere. (And this can be accomplished, but it's a somewhat monumental task to design and implement.)
There's no free ride here, I don't think. Again, they're solvable problems but they're expensive to solve.
As for fire safety and egresses it basically begins outside the unit, so a hotel or boarding house and a office space actually have the same egress rules while a 3 bedroom is a death trap.