Say you have an average size house, with a 2-car garage on the side. You decide to change that garage into a small apartment for renting. You need to add a wall or two, add insulation, build up a kitchen area (with proper water and power) and a bathroom.
Imagine how much that would cost you for that single apartment. Now multiply that by, say, 50, to convert a large office building into 50 residential rental units. Even with economies of scale, that’s still going to cost millions…