Would you live in an apartment 15 feet wide and 50 feet deep? No. That's why most office buildings dont work
You have to redo all the mechanical, plumbing, electric, and then entirely refinish the building. Its literally cheaper to build new than deal with the nightmare that is to convert an office.
I work in CRE - every single office to multi deal i have seen the developer has war stories. And the units are 40-100% more expensive in cost than other apartments.
Office buildings are like an infectious disease. You could let them die. But if you have an urban infill city environment and more and more office properties become ghost buildings, then that disease will spread. All of the restaurants and other businesses nearby that rely on the foot traffic from office folk, they will die. You will see more riff raff. More homeless. More crime.
Many office assets need to go to 0 in asset value before the private market is willing to take the risk of those conversions.
Remember, loans are close to 10% for construction. And equity returns are higher.