A few core reasons (im an urban designer and deal with this sort of thing fairly regularly)
Residential and modern office typically have very different floorplate requirements. So often dont sit comforatably over each other.
Mixing residential and commercial tends to make building operation an issue - who pays or chooses what to spend on when there are building wide issues (leaks, structure, lifts, noise complaints, etc)? What happens when the office owner wants a complete refurb? Office owners traditionally have liked to retain ownership to allow demolition and reconstruction in after 40 years or so.
Many high grade offices tend to be picky and not like looking out the window at peoples laundry in the opposite building. Nor have the lobby of low cost apartment housing in the side of the same building.
Adding more levels isnt always an economic thing to do for developers since time in construction (and hence time the developer has borrowed money but not recieved payments) is a key issue.
At a strategic level often true 'office cores' can be constrained and need to be protected from erosion.
Residential and modern office typically have very different floorplate requirements. So often dont sit comforatably over each other.
Mixing residential and commercial tends to make building operation an issue - who pays or chooses what to spend on when there are building wide issues (leaks, structure, lifts, noise complaints, etc)? What happens when the office owner wants a complete refurb? Office owners traditionally have liked to retain ownership to allow demolition and reconstruction in after 40 years or so.
Many high grade offices tend to be picky and not like looking out the window at peoples laundry in the opposite building. Nor have the lobby of low cost apartment housing in the side of the same building.
Adding more levels isnt always an economic thing to do for developers since time in construction (and hence time the developer has borrowed money but not recieved payments) is a key issue.
At a strategic level often true 'office cores' can be constrained and need to be protected from erosion.