It’s not that there might not be plumbing, it’s that there is zero plumbing in most office buildings aside from one clustered section for floor where there’s 5 to 10 toilets for each gender.
On top of that, you have completely different mechanical systems. An office building for instance may have one single mechanical system for the entire building, whereas an apartment would need separate mechanical systems for each individual apartment.
Then you have the kitchens, bedrooms and interior partition walls that are vastly different than an office building, plus the requirements for exterior windows which precludes larger office buildings with deeper floor plates from being converted at all without demolishing the interior portion of the building. Curtain wall systems that may or may not be compatible between an office and residential building (non-operating windows)… Not to mention the stair and elevator systems are probably not going to work either.
So in the end you’re probably looking at gutting the building down to the structure and removing every piece of the building including the outer envelope, roof, all of the electrical plumbing and mechanical systems… In the end it may or may not be cheaper just to build a new building from the ground up.
Source: am architect. And yes, I have done conversions like this in the past.