So what makes converting existing structures to residential use harder than building new residential buildings on empty land? I view this as an "outrunning the

So what makes converting existing structures to residential use harder than building new residential buildings on empty land? I view this as an "outrunning the bear" situation. I.e. you don't need to run faster than the bear, just run faster than your poor friend. The cost to convert an office tower into apartments is high I'm sure, but is it higher than building a similarly-sized apartment tower from scratch? A lot of money goes into the foundation works and soils engineering.