Half of the issue is only a problem under capitalism, but the other half is real.

Yes, it may be more work and require slightly more expense to retrofit and remodel an older home into being as efficient as an average more recent home than to just slap up a builder special house. It will never be as efficient as a home designed for maximum efficiency over the whole service lifetime of the home, but the reduction in resources needed and ability to reuse and recycle parts from homes that are being retrofitted and remodeled are major advantages, as is the quality increase from doing it right instead of slapping up McMansion shit.

The actual problem is that most of the vacant homes in the US are in the middle of nowhere, either vacation homes or in extremely rural areas, very far from everything you need to work and live. Most abandoned homes are abandoned for a reason, not just because they're kinda crappy.

Refitting office buildings into housing is a whole different story, and many genuinely would need to be completely torn back to the bare frame or just demolished to convert them into livable housing, not only because of building codes but because the structures are designed very differently and would likely result in many homes without any windows at all unless you make super weirdly-shaped apartments. Water, HVAC, and electrical are also concerns, as they can only be controlled centrally which a lot of people don't like.